Ensure literal pathspecs are used for LastCommitForPath

By default, Git allows the pathspec to have glob patterns (https://css-tricks.com/git-pathspecs-and-how-to-use-them), but the LastCommitForPath RPC only uses the path argument as a literal. This can be done via the --literal-pathspecs argument (https://git-scm.com/docs/git#Documentation/git.txt---literal-pathspecs) or by adding (:literal) before the pathspec. This commit does the former because it seems clearer.

Closes #2857 (closed)