rhg: Make `files` work on repo-relative paths when possible When the current directory is outside of the repository we need to turn everything into absolute filesystem paths in order to compute correct relative paths. This was previously done unconditionally, but is not necessary when the current directory is inside the repository. With this change `rhg files > /dev/null` at the root of a mozilla-central snapshot goes from ~150 ms to ~70 ms. My repository is located at a somewhat long path though (93 bytes). The effect may not be as pronounced at a shorter path. Differential Revision: https://phab.mercurial-scm.org/D10200