Define Xgit.Tag, which describes a tag object in memory.

[scouten]

Changes in This Pull Request

This object will support upcoming plumbing commands related to tags.

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.
• 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 #267 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #267      +/-   ##
==========================================
+ Coverage   99.85%   99.85%   +<.01%     
==========================================
  Files          27       28       +1     
  Lines        1348     1353       +5     
==========================================
+ Hits         1346     1351       +5     
  Misses          2        2

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

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