A couple of gitfs fixes

1. Clear out the gitfs file_lists cache on master start. These caches expire after 30 secs, but there could be edge cases where someone makes a change to the configuration that would change the available files, and then quickly restarts the master and executes something that uses the fileserver. The first commit just future-proofs against that by clearing file_lists caches on master start.

2. Strip the trailing slash from the remote's "root" config param, whether it is coming from the global ``gitfs_root`` or from a per-config override. The trailing slash messes with file lookups and causes files not to be available. This fixes #13185.