gitk: Make "show origin of this line" work on fake commits

gitk: Make "show origin of this line" work on fake commits

This makes the "Show origin of this line" menu item work correctly
on the fake commits that gitk shows for local uncommitted changes.
With the fake commit for changes that aren't checked in to the index,
we can actually get a 3-way diff shown, which means we might have to
blame either the parent or the commit being merged in (which we get
from .git/MERGE_HEAD).

If the parent is the fake commit which shows the changes that have
been checked in to the index, then we need to get the SHA1 of the blob
for the version of the file that is in the index, then use git cat-file
blob to get the contents of the blob, and give that to git blame with --contents - so that git blame will do the blame on the index version
of the file.  In that case, we might get the all-zeroes SHA1 back from
git blame, meaning that the line is new in the index version of the
file, so then we have to use $nullid2 (the pseudo-SHA1 of the fake
commit for the checked-in changes).

Signed-off-by: Paul Mackerras <paulus@samba.org>