Implement Xgit.Repository.Plumbing.get_symbolic_ref/2.

[scouten]

Changes in This Pull Request

This is an API equivalent to the one-argument form of git symbolic-ref.

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. n/a

[codecov]

Codecov Report

Merging #261 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #261   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          27     27           
  Lines        1334   1341    +7     
=====================================
+ Hits         1334   1341    +7

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

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