Add annotation supporting methods to GitArchiver

[devdanzin]

This PR adds GitArchiver.is_data_outdated() (which checks whether a file in the working directory and on a given revision have diverged) and GitArchiver..get_file_contents() (which allows fetching the contents of a file in a given revision) to support source annotation. Also adds simple tests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.89%. Comparing base (2590691) to head (3ed5f54).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #223      +/-   ##
==========================================
+ Coverage   95.86%   95.89%   +0.02%     
==========================================
  Files          25       25              
  Lines        1403     1413      +10     
  Branches      296      296              
==========================================
+ Hits         1345     1355      +10     
  Misses         33       33              
  Partials       25       25              

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