Implement Xgit.Plumbing.UpdateRef.

[scouten]

Changes in This Pull Request

This is an API equivalent to git update-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.

[codecov]

Codecov Report

Merging #242 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #242      +/-   ##
==========================================
+ Coverage   99.92%   99.92%   +<.01%     
==========================================
  Files          36       37       +1     
  Lines        1310     1325      +15     
==========================================
+ Hits         1309     1324      +15     
  Misses          1        1

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2719486...5a954c2. Read the comment docs.