patch: add config knob for displaying the index header
This config knob can take an integer between 0 and 40 or a
keyword ('none', 'short', 'full') to control the length of hash to
output. It will display diffs with the git index header as such,
diff --git a/mercurial/mdiff.py b/mercurial/mdiff.py
index 112edf7..d6b52c5 100644
We'll put this in the experimental section for now.
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.