diff: use filespec path to set up tempfiles for ext-diff

When we're going to run an external diff, we have to make the contents
of the pre- and post-images available either by dumping them to a
tempfile, or by pointing at a valid file in the worktree. The logic of
this is all handled by prepare_temp_file(), and we just pass in the
filename and the diff_filespec.

But there's a gotcha here. The "filename" we have is a logical filename
and not necessarily a path on disk or in the repository. This matters in
at least one case: when using "--relative", we may have a name like
"foo", even though the file content is found at "subdir/foo". As a
result, we look for the wrong path, fail to find "foo", and claim that
the file has been deleted (passing "/dev/null" to the external diff,
rather than the correct worktree path).

We can fix this by passing the pathname from the diff_filespec, which
should always be a full repository path (and that's what we want even if
reusing a worktree file, since we're always operating from the top-level
of the working tree).

The breakage seems to go all the way back to cd676a5136 (diff
--relative: output paths as relative to the current subdirectory,
2008-02-12). As far as I can tell, before then "name" would always have
been the same as the filespec's "path".

There are two related cases I looked at that aren't buggy:

  1. the only other caller of prepare_temp_file() is run_textconv(). But
     it always passes the filespec's path field, so it's OK.

  2. I wondered if file renames/copies might cause similar confusion.
     But they don't, because run_external_diff() receives two names in
     that case: "name" and "other", which correspond to the two sides of
     the diff. And we did correctly pass "other" when handling the
     post-image side. Barring the use of "--relative", that would always
     match "two->path", the path of the second filespec (and the rename
     destination).

So the only bug is just the interaction with external diff drivers and
--relative.

Reported-by: Carl Baldwin <carl@ecbaldwin.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -4281,7 +4281,7 @@ static void add_external_diff_name(struct repository *r,
 				   const char *name,
 				   struct diff_filespec *df)
 {
-	struct diff_tempfile *temp = prepare_temp_file(r, name, df);
+	struct diff_tempfile *temp = prepare_temp_file(r, df->path, df);
 	strvec_push(argv, temp->name);
 	strvec_push(argv, temp->hex);
 	strvec_push(argv, temp->mode);