commitService: implement CountCommits (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-patch2 in 1 minute and 24 seconds
Status Job ID Name Coverage
  Main
passed #116762
tests-current

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  Compat
passed #116763
check-sdist

00:00:28

passed #116765
tests-hg-default

00:00:44

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passed #116764
tests-hg-stable

00:00:52

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