Implement Xgit.PackReader.

[scouten]

Changes in This Pull Request

This module provides read access to pack and pack index file pairs.

Checklist

IN PROGRESS, WILL COMPLETE LATER

• 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 #319 into master will decrease coverage by 0.88%.
The diff coverage is 90.86%.

@@             Coverage Diff             @@
##            master     #319      +/-   ##
===========================================
- Coverage   100.00%   99.11%   -0.89%     
===========================================
  Files           32       33       +1     
  Lines         1728     1914     +186     
===========================================
+ Hits          1728     1897     +169     
- Misses           0       17      +17     

Impacted Files	Coverage Δ
lib/xgit/pack_reader.ex	90.86% <90.86%> (ø)

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