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>

builtin/diff.c

@@ -90,7 +90,8 @@ static int builtin_diff_b_f(struct rev_info *revs,
 		     blob[0]->mode, canon_mode(st.st_mode),
 		     &blob[0]->item->oid, &null_oid,
 		     1, 0,
-		     path, path);
+		     blob[0]->path ? blob[0]->path : path,
+		     path);
 	diffcore_std(&revs->diffopt);
 	diff_flush(&revs->diffopt);
 	return 0;

t/t4063-diff-blobs.sh

@@ -81,11 +81,16 @@ test_expect_success 'diff blob against file' '
 test_expect_success 'index of blob-file diff' '
 	check_index $sha1_one $sha1_two
 '
-test_expect_failure 'blob-file diff uses filename as paths' '
+test_expect_success 'blob-file diff uses filename as paths' '
 	check_paths one two
 '
 test_expect_success FILEMODE 'blob-file diff shows mode change' '
 	check_mode 100644 100755
 '
 
+test_expect_success 'blob-file diff prefers filename to sha1' '
+	run_diff $sha1_one two &&
+	check_paths two two
+'
+
 test_done