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.
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https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
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273ce12ad8f1` will report some style violations on a very old commit).