CommitService.LastCommitForPath: initial implementation

As explained in the comment, we must really use
`file()` here.
4 jobs for topic/default/last-commit-for-path in 2 minutes and 11 seconds (queued for 1 second)
Status Job ID Name Coverage
  Main
passed #118905
tests-current

00:01:23

100.0%
 
  Compat
passed #118906
check-sdist

00:00:26

passed #118908
tests-hg-default

00:00:47

100.0%
passed #118907
tests-hg-stable

00:00:45

100.0%