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/sergi/go-diff/AUTHORS

@@ -0,0 +1,25 @@
+# This is the official list of go-diff authors for copyright purposes.
+# This file is distinct from the CONTRIBUTORS files.
+# See the latter for an explanation.
+
+# Names should be added to this file as
+#	Name or Organization <email address>
+# The email address is not required for organizations.
+
+# Please keep the list sorted.
+
+Danny Yoo <dannyyoo@google.com>
+James Kolb <jkolb@google.com>
+Jonathan Amsterdam <jba@google.com>
+Markus Zimmermann <markus.zimmermann@nethead.at> <markus.zimmermann@symflower.com> <zimmski@gmail.com>
+Matt Kovars <akaskik@gmail.com>
+Örjan Persson <orjan@spotify.com>
+Osman Masood <oamasood@gmail.com>
+Robert Carlsen <rwcarlsen@gmail.com>
+Rory Flynn <roryflynn@users.noreply.github.com>
+Sergi Mansilla <sergi.mansilla@gmail.com>
+Shatrugna Sadhu <ssadhu@apcera.com>
+Shawn Smith <shawnpsmith@gmail.com>
+Stas Maksimov <maksimov@gmail.com>
+Tor Arvid Lund <torarvid@gmail.com>
+Zac Bergquist <zbergquist99@gmail.com>