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/cloudflare/circl/dh/x25519/doc.go

@@ -0,0 +1,20 @@
+/*
+Package x25519 provides Diffie-Hellman functions as specified in RFC-7748.
+
+Validation of public keys.
+
+The Diffie-Hellman function, as described in RFC-7748 [1], works for any
+public key. However, if a different protocol requires contributory
+behaviour [2,3], then the public keys must be validated against low-order
+points [3,4]. To do that, the Shared function performs this validation
+internally and returns false when the public key is invalid (i.e., it
+is a low-order point).
+
+References:
+ - [1] RFC7748 by Langley, Hamburg, Turner (https://rfc-editor.org/rfc/rfc7748.txt)
+ - [2] Curve25519 by Bernstein (https://cr.yp.to/ecdh.html)
+ - [3] Bernstein (https://cr.yp.to/ecdh.html#validate)
+ - [4] Cremers&Jackson (https://eprint.iacr.org/2019/526)
+
+*/
+package x25519