[API BREAKING] Add reference operations to Repository interface.

[scouten]

Checklist

• This PR represents a single feature, fix, or change.
• All applicable changes have been documented.
• There is test coverage for all changes.
• All cases where a literal value is returned use the cover macro to force code coverage.
• Any code ported from jgit maintains all existing copyright and license notices. n/a
• If new files are ported from jgit, the path to the corresponding file(s) is included in the header comment. n/a
• Any TO DO items (or similar) have been entered as GitHub issues and the link to that issue has been included in a comment.

[codecov]

Codecov Report

Merging #222 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #222   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          36     36           
  Lines        1197   1249   +52     
=====================================
+ Hits         1197   1249   +52

Impacted Files	Coverage Δ
lib/xgit/repository.ex	100% <100%> (ø)	⬆️
lib/xgit/repository/in_memory.ex	100% <100%> (ø)	⬆️
lib/xgit/repository/on_disk.ex	100% <100%> (ø)	⬆️

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