Fix 500 errors with filenames that contain glob characters

This fixes errors that occur when filenames such as :wq are used in the repository.

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 now supports a literal_pathspec to disable this (gitaly!2285 (merged)). We now use this whenever we look up specific files in the repository.

Closes gitaly#2857 (closed)