diff-no-index: align D/F handling with that of normal Git

When a commit changes a path P that used to be a file to a directory
and creates a new path P/X in it, "git show" would say that file P
was removed and file P/X was created for such a commit.

However, if we compare two directories, D1 and D2, where D1 has a
file D1/P in it and D2 has a directory D2/P under which there is a
file D2/P/X, and ask "git diff --no-index D1 D2" to show their
differences, we simply get a refusal "file/directory conflict".

Surely, that may be what GNU diff does, but we can do better and it
is easy to do so.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff-no-index.c

@@ -97,8 +97,27 @@ static int queue_diff(struct diff_options *o,
 	if (get_mode(name1, &mode1) || get_mode(name2, &mode2))
 		return -1;
 
-	if (mode1 && mode2 && S_ISDIR(mode1) != S_ISDIR(mode2))
-		return error("file/directory conflict: %s, %s", name1, name2);
+	if (mode1 && mode2 && S_ISDIR(mode1) != S_ISDIR(mode2)) {
+		struct diff_filespec *d1, *d2;
+
+		if (S_ISDIR(mode1)) {
+			/* 2 is file that is created */
+			d1 = noindex_filespec(NULL, 0);
+			d2 = noindex_filespec(name2, mode2);
+			name2 = NULL;
+			mode2 = 0;
+		} else {
+			/* 1 is file that is deleted */
+			d1 = noindex_filespec(name1, mode1);
+			d2 = noindex_filespec(NULL, 0);
+			name1 = NULL;
+			mode1 = 0;
+		}
+		/* emit that file */
+		diff_queue(&diff_queued_diff, d1, d2);
+
+		/* and then let the entire directory be created or deleted */
+	}
 
 	if (S_ISDIR(mode1) || S_ISDIR(mode2)) {
 		struct strbuf buffer1 = STRBUF_INIT;

t/t4053-diff-no-index.sh

@@ -77,4 +77,16 @@ test_expect_success 'diff D F and diff F D' '
 	)
 '
 
+test_expect_success 'turning a file into a directory' '
+	(
+		cd non/git &&
+		mkdir d e e/sub &&
+		echo 1 >d/sub &&
+		echo 2 >e/sub/file &&
+		printf "D\td/sub\nA\te/sub/file\n" >expect &&
+		test_must_fail git diff --no-index --name-status d e >actual &&
+		test_cmp expect actual
+	)
+'
+
 test_done