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/diffmatchpatch/operation_string.go

@@ -0,0 +1,17 @@
+// Code generated by "stringer -type=Operation -trimprefix=Diff"; DO NOT EDIT.
+
+package diffmatchpatch
+
+import "fmt"
+
+const _Operation_name = "DeleteEqualInsert"
+
+var _Operation_index = [...]uint8{0, 6, 11, 17}
+
+func (i Operation) String() string {
+	i -= -1
+	if i < 0 || i >= Operation(len(_Operation_index)-1) {
+		return fmt.Sprintf("Operation(%d)", i+-1)
+	}
+	return _Operation_name[_Operation_index[i]:_Operation_index[i+1]]
+}