Use argcomplete to auto-generate autocompletion

[soraxas]

The use of argcomplete is much more robust and it works on many different shells. This also makes the auto-complete very much up-to-date when we change gita in the future.

The only downside is that gita now depends on argcomplete, which I think is reasonable dependency.

[nosarthur]

please keep the original autocompletion scripts as alternatives

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (2b70e72) 86.27% compared to head (6b8d4d2) 86.43%.

❗ Current head 6b8d4d2 differs from pull request most recent head 9963986. Consider uploading reports for the commit 9963986 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #269      +/-   ##
==========================================
+ Coverage   86.27%   86.43%   +0.16%     
==========================================
  Files           5        5              
  Lines         794      796       +2     
==========================================
+ Hits          685      688       +3     
+ Misses        109      108       -1     

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[soraxas]

Yep, no problem