diff: use blob path for blob/file diffs

diff: use blob path for blob/file diffs

When we diff a blob against a working tree file like:

  git diff HEAD:Makefile Makefile

we always use the working tree filename for both sides of
the diff. In most cases that's fine, as the two would be the
same anyway, as above. And until recently, we used the
"name" for the blob, not the path, which would have the
messy "HEAD:" on the beginning.

But when they don't match, like:

  git diff HEAD:old_path new_path

it makes sense to show both names.

This patch uses the blob's path field if it's available, and
otherwise falls back to using the filename (in preference to
the blob's name, which is likely to be garbage like a raw
sha1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>