RHGitaly implement RefService.RefExists

According to the statistics break down taken just now on foss.heptapod.net, this is the second-most frequent method:

TOTAL Requests: 679185
Breakdown:
  47.5% FindCommitRequest (count=322444)
   9.4% RefExistsRequest (count=64032)
   7.5% TreeEntryRequest (count=50906)

(since 2023-04-29 18:47:13,342)

If should be even easier to implement than FindCommit, because it is a direct application of #126 (closed). One might also say that it is the one focusing the most the risk of depending on the GitLab state files.

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