Correct diff header formatting

Correct diff header formatting

When adding or deleting a file, we shouldn't ever prefix /dev/null
with the a/ or b/ prefixes.  Doing so is a mistake and confuses a
patch parser which handles /dev/null magically, while a/dev/null is
a file called null in the dev directory of the project.

Also when adding or deleting the "diff --git" line has the "real"
path on both sides, so we should see the following when adding the
file called foo:

  diff --git a/foo b/foo
  --- /dev/null
  +++ b/foo

The --- and +++ lines do not appear in a pure rename or copy delta,
C Git diff seems to omit these, so we now omit them as well.  We also
omit the index line when the ObjectIds are exactly equal.

Change-Id: Ic46892dea935ee8bdee29088aab96307d7ec6d3d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>