codeintel: Add CommitGraph to gitserver client

Created by: efritz

This PR adds a method to the codeintel gitserver client that requests the parents of each commit known to the gitserver's clone. This will be used to construct a full commit graph for a repository.

This is part of a larger effort towards https://github.com/sourcegraph/sourcegraph/issues/12098.

Created by: codecov[bot]

Codecov Report

Merging #12404 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #12404   +/-   ##
=======================================
  Coverage   48.86%   48.86%           
=======================================
  Files        1400     1400           
  Lines       78531    78531           
  Branches     6407     6407           
=======================================
  Hits        38371    38371           
  Misses      36708    36708           
  Partials     3452     3452           

Flag	Coverage Δ
#go	51.93% <0.00%> (ø)
#integration	24.25% <0.00%> (ø)
#storybook	12.63% <0.00%> (ø)
#typescript	40.41% <0.00%> (ø)
#unit	47.06% <0.00%> (ø)

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