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/github.com/Microsoft/go-winio/syscall.go

@@ -0,0 +1,3 @@
+package winio
+
+//go:generate go run golang.org/x/sys/windows/mkwinsyscall -output zsyscall_windows.go file.go pipe.go sd.go fileinfo.go privilege.go backup.go hvsock.go