Global ConfigParser

[benthayer]

fixes#775

We can instantiate a GitConfigParser with no files specified, if we do so, then it'll figure out where those files are. It does this by either having a config_level specified to it or, in the case of a config reader, by default we might want to read from all configuration levels, just like is currently the case in Repo.config_writer(), so we iterate through all of them (except repository level) and add all files to the parser.

If the repository level is specified, we raise a unique ValueError telling the user to use the Repo object instead.

[codecov-io]

Codecov Report

Merging #950 into master will decrease coverage by 0.01%.
The diff coverage is 22.72%.

@@ Coverage Diff @@## master #950 +/- ## ========================================= - Coverage 93.52% 93.5% -0.02%  ========================================= Files 59 59 Lines 9677 9915 +238 ========================================= + Hits 9050 9271 +221 - Misses 627 644 +17

Impacted Files	Coverage Δ
git/config.py	88.4% <22.72%> (-3.87%)	⬇️
git/objects/blob.py	100% <0%> (ø)	⬆️
git/refs/remote.py	100% <0%> (ø)	⬆️
git/test/test_diff.py	100% <0%> (ø)	⬆️
git/test/performance/test_streams.py	100% <0%> (ø)	⬆️
git/test/test_tree.py	100% <0%> (ø)	⬆️
git/test/test_exc.py	100% <0%> (ø)	⬆️
git/test/test_docs.py	99.6% <0%> (ø)	⬆️
git/test/test_refs.py	99.18% <0%> (ø)	⬆️
git/test/test_config.py	98.8% <0%> (ø)	⬆️
... and 27 more

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[Byron]

Thanks a lot for your contribution, it's looking great! I particularly like that the change is implemented without breaking compatibility.

There are just two minor things I saw when reviewing, please let me know what you think about them.

[Byron]

Thanks a lot for your effort! It's a great improvement!

[benthayer]

Happy to help!