feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/modules.txt

@@ -1,3 +1,30 @@
+# github.com/Microsoft/go-winio v0.5.2
+## explicit; go 1.13
+github.com/Microsoft/go-winio
+github.com/Microsoft/go-winio/pkg/guid
+# github.com/ProtonMail/go-crypto v0.0.0-20220824120805-4b6e5c587895
+## explicit; go 1.13
+github.com/ProtonMail/go-crypto/bitcurves
+github.com/ProtonMail/go-crypto/brainpool
+github.com/ProtonMail/go-crypto/eax
+github.com/ProtonMail/go-crypto/internal/byteutil
+github.com/ProtonMail/go-crypto/ocb
+github.com/ProtonMail/go-crypto/openpgp
+github.com/ProtonMail/go-crypto/openpgp/aes/keywrap
+github.com/ProtonMail/go-crypto/openpgp/armor
+github.com/ProtonMail/go-crypto/openpgp/ecdh
+github.com/ProtonMail/go-crypto/openpgp/ecdsa
+github.com/ProtonMail/go-crypto/openpgp/eddsa
+github.com/ProtonMail/go-crypto/openpgp/elgamal
+github.com/ProtonMail/go-crypto/openpgp/errors
+github.com/ProtonMail/go-crypto/openpgp/internal/algorithm
+github.com/ProtonMail/go-crypto/openpgp/internal/ecc
+github.com/ProtonMail/go-crypto/openpgp/internal/encoding
+github.com/ProtonMail/go-crypto/openpgp/packet
+github.com/ProtonMail/go-crypto/openpgp/s2k
+# github.com/acomagu/bufpipe v1.0.3
+## explicit; go 1.12
+github.com/acomagu/bufpipe
 # github.com/alecthomas/chroma v0.10.0
 ## explicit; go 1.13
 github.com/alecthomas/chroma
@@ -34,6 +61,20 @@ github.com/alecthomas/chroma/lexers/y
 github.com/alecthomas/chroma/lexers/z
 github.com/alecthomas/chroma/quick
 github.com/alecthomas/chroma/styles
+# github.com/cloudflare/circl v1.2.0
+## explicit; go 1.16
+github.com/cloudflare/circl/dh/x25519
+github.com/cloudflare/circl/dh/x448
+github.com/cloudflare/circl/ecc/goldilocks
+github.com/cloudflare/circl/internal/conv
+github.com/cloudflare/circl/internal/sha3
+github.com/cloudflare/circl/math
+github.com/cloudflare/circl/math/fp25519
+github.com/cloudflare/circl/math/fp448
+github.com/cloudflare/circl/math/mlsbset
+github.com/cloudflare/circl/sign
+github.com/cloudflare/circl/sign/ed25519
+github.com/cloudflare/circl/sign/ed448
 # github.com/davecgh/go-spew v1.1.1
 ## explicit
 github.com/davecgh/go-spew/spew
@@ -44,22 +85,122 @@ github.com/dlclark/regexp2/syntax
 # github.com/docopt/docopt-go v0.0.0-20180111231733-ee0de3bc6815
 ## explicit
 github.com/docopt/docopt-go
-# github.com/kr/text v0.2.0
+# github.com/emirpasic/gods v1.18.1
+## explicit; go 1.2
+github.com/emirpasic/gods/containers
+github.com/emirpasic/gods/lists
+github.com/emirpasic/gods/lists/arraylist
+github.com/emirpasic/gods/trees
+github.com/emirpasic/gods/trees/binaryheap
+github.com/emirpasic/gods/utils
+# github.com/go-git/gcfg v1.5.0
 ## explicit
+github.com/go-git/gcfg
+github.com/go-git/gcfg/scanner
+github.com/go-git/gcfg/token
+github.com/go-git/gcfg/types
+# github.com/go-git/go-billy/v5 v5.3.1
+## explicit; go 1.13
+github.com/go-git/go-billy/v5
+github.com/go-git/go-billy/v5/helper/chroot
+github.com/go-git/go-billy/v5/helper/polyfill
+github.com/go-git/go-billy/v5/memfs
+github.com/go-git/go-billy/v5/osfs
+github.com/go-git/go-billy/v5/util
+# github.com/go-git/go-git/v5 v5.4.2
+## explicit; go 1.13
+github.com/go-git/go-git/v5
+github.com/go-git/go-git/v5/config
+github.com/go-git/go-git/v5/internal/revision
+github.com/go-git/go-git/v5/internal/url
+github.com/go-git/go-git/v5/plumbing
+github.com/go-git/go-git/v5/plumbing/cache
+github.com/go-git/go-git/v5/plumbing/color
+github.com/go-git/go-git/v5/plumbing/filemode
+github.com/go-git/go-git/v5/plumbing/format/config
+github.com/go-git/go-git/v5/plumbing/format/diff
+github.com/go-git/go-git/v5/plumbing/format/gitignore
+github.com/go-git/go-git/v5/plumbing/format/idxfile
+github.com/go-git/go-git/v5/plumbing/format/index
+github.com/go-git/go-git/v5/plumbing/format/objfile
+github.com/go-git/go-git/v5/plumbing/format/packfile
+github.com/go-git/go-git/v5/plumbing/format/pktline
+github.com/go-git/go-git/v5/plumbing/object
+github.com/go-git/go-git/v5/plumbing/protocol/packp
+github.com/go-git/go-git/v5/plumbing/protocol/packp/capability
+github.com/go-git/go-git/v5/plumbing/protocol/packp/sideband
+github.com/go-git/go-git/v5/plumbing/revlist
+github.com/go-git/go-git/v5/plumbing/storer
+github.com/go-git/go-git/v5/plumbing/transport
+github.com/go-git/go-git/v5/plumbing/transport/client
+github.com/go-git/go-git/v5/plumbing/transport/file
+github.com/go-git/go-git/v5/plumbing/transport/git
+github.com/go-git/go-git/v5/plumbing/transport/http
+github.com/go-git/go-git/v5/plumbing/transport/internal/common
+github.com/go-git/go-git/v5/plumbing/transport/server
+github.com/go-git/go-git/v5/plumbing/transport/ssh
+github.com/go-git/go-git/v5/storage
+github.com/go-git/go-git/v5/storage/filesystem
+github.com/go-git/go-git/v5/storage/filesystem/dotgit
+github.com/go-git/go-git/v5/storage/memory
+github.com/go-git/go-git/v5/utils/binary
+github.com/go-git/go-git/v5/utils/diff
+github.com/go-git/go-git/v5/utils/ioutil
+github.com/go-git/go-git/v5/utils/merkletrie
+github.com/go-git/go-git/v5/utils/merkletrie/filesystem
+github.com/go-git/go-git/v5/utils/merkletrie/index
+github.com/go-git/go-git/v5/utils/merkletrie/internal/frame
+github.com/go-git/go-git/v5/utils/merkletrie/noder
+# github.com/imdario/mergo v0.3.13
+## explicit; go 1.13
+github.com/imdario/mergo
+# github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99
+## explicit
+github.com/jbenet/go-context/io
+# github.com/kevinburke/ssh_config v1.2.0
+## explicit
+github.com/kevinburke/ssh_config
 # github.com/mattn/go-isatty v0.0.16
 ## explicit; go 1.15
 github.com/mattn/go-isatty
 # github.com/mitchellh/go-homedir v1.1.0
 ## explicit
 github.com/mitchellh/go-homedir
-# github.com/niemeyer/pretty v0.0.0-20200227124842-a10e7caefd8e
+# github.com/sergi/go-diff v1.2.0
 ## explicit; go 1.12
+github.com/sergi/go-diff/diffmatchpatch
+# github.com/xanzy/ssh-agent v0.3.2
+## explicit; go 1.16
+github.com/xanzy/ssh-agent
+# golang.org/x/crypto v0.0.0-20220826181053-bd7e27e6170d
+## explicit; go 1.17
+golang.org/x/crypto/blowfish
+golang.org/x/crypto/cast5
+golang.org/x/crypto/chacha20
+golang.org/x/crypto/curve25519
+golang.org/x/crypto/curve25519/internal/field
+golang.org/x/crypto/ed25519
+golang.org/x/crypto/internal/alias
+golang.org/x/crypto/internal/poly1305
+golang.org/x/crypto/ssh
+golang.org/x/crypto/ssh/agent
+golang.org/x/crypto/ssh/internal/bcrypt_pbkdf
+golang.org/x/crypto/ssh/knownhosts
+# golang.org/x/net v0.0.0-20220826154423-83b083e8dc8b
+## explicit; go 1.17
+golang.org/x/net/context
+golang.org/x/net/internal/socks
+golang.org/x/net/proxy
 # golang.org/x/sys v0.0.0-20220825204002-c680a09ffe64
 ## explicit; go 1.17
+golang.org/x/sys/cpu
+golang.org/x/sys/execabs
 golang.org/x/sys/internal/unsafeheader
 golang.org/x/sys/unix
-# gopkg.in/check.v1 v1.0.0-20200227125254-8fa46927fb4f
+golang.org/x/sys/windows
+# gopkg.in/warnings.v0 v0.1.2
 ## explicit
+gopkg.in/warnings.v0
 # gopkg.in/yaml.v1 v1.0.0-20140924161607-9f9df34309c0
 ## explicit
 gopkg.in/yaml.v1