hgweb: use patch.diff() to make sensible diffs (
issue1223,
issue1258)
This gets us git diffs when enabled and doesn't pick erratic file parents.
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0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
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0 files updated, 0 files merged, 0 files removed, 0 files unresolved
adding b
created new head