[RFC/PATCH] align D/F handling of "diff --no-index" with that of normal Git

When a commit changes a path P that used to be a file to a directory
and create 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".

The "diff --no-index" implementation has an underlying machinery
that can make it more in line with the normal Git if it wanted to,
but somehow it is not being exercised.  The only thing we need to
do, when we see a file P and a directory P/ (or the other way
around) is to show the removal of a file P and then pretend as if we
are comparing nothing with a whole directory P/, as the code is
fully prepared to express a creation of everything in a directory
(and if the comparison is between a directory P/ and a file P, then
show the creation of the file and then let the existing code remove
everything in P/).

Signed-off-by: Junio C Hamano <redacted>
---
 diff-no-index.c | 23 +++++++++++++++++++++--
 1 file changed, 21 insertions(+), 2 deletions(-)

diff --git a/diff-no-index.c b/diff-no-index.c
index 265709b..52e9546 100644
--- a/diff-no-index.c
+++ b/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, mode2);
+			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 created or deleted */
+	}
 
 	if (S_ISDIR(mode1) || S_ISDIR(mode2)) {
 		struct strbuf buffer1 = STRBUF_INIT;