commands/path.go: introduce trimCurrentPrefix to remove "./"

There exist cases within Git LFS where we would like to perform a
"pseudo-clean" on a given filepath. It should behave similarly to a call
to filepath.Clean, but should only remove the beginning "./" (macOS,
*nix) or ".\" (Windows, etc).

This patch introduces trimCurrentPrefix, which removes such prefixes.
It will be used in subsequent patches to perform prefix-independent
comparison against arguments to "git lfs track" and "git lfs untrack"
and the contents of a repository's .gitattributes file.

Since we assume that the contents of ".gitattributes" are written on
multiple platforms, each with their own platform-specific prefixes,
trimCurrentPrefix is instructed to remove _all_ prefixes, not just the
current platforms.

commands/path.go

@@ -12,6 +12,18 @@ func gitLineEnding(git env) string {
 	}
 }
 
+const (
+	windowsPrefix = `.\`
+	nixPrefix     = `./`
+)
+
+func trimCurrentPrefix(p string) string {
+	if strings.HasPrefix(p, windowsPrefix) {
+		return strings.TrimPrefix(p, windowsPrefix)
+	}
+	return strings.TrimPrefix(p, nixPrefix)
+}
+
 type env interface {
 	Get(string) (string, bool)
 }