CommitService: implement FindCommits (partially for now)

For now, this doesn't support all lookups, but we plan to
complete it by HGitaly1 milestone.
4 jobs for topic/default/khanchi-hgitaly-patch3 in 2 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Main
passed #118776
tests-current

00:00:31

100.0%
 
  Compat
passed #118777
check-sdist

00:00:18

passed #118779
tests-hg-default

00:00:37

100.0%
passed #118778
tests-hg-stable

00:01:37

100.0%