diffcore-rename: don't consider unmerged path as source

Since e9c8409 (diff-index --cached --raw: show tree entry on the LHS for
unmerged entries., 2007-01-05), an unmerged entry should be detected by
using DIFF_PAIR_UNMERGED(p), not by noticing both one and two sides of
the filepair records mode=0 entries. However, it forgot to update some
parts of the rename detection logic.

This only makes difference in the "diff --cached" codepath where an
unmerged filepair carries information on the entries that came from the
tree.  It probably hasn't been noticed for a long time because nobody
would run "diff -M" during a conflict resolution, but "git status" uses
rename detection when it internally runs "diff-index" and "diff-files"
and gives nonsense results.

In an unmerged pair, "one" side can have a valid filespec to record the
tree entry (e.g. what's in HEAD) when running "diff --cached". This can
be used as a rename source to other paths in the index that are not
unmerged. The path that is unmerged by definition does not have the
final content yet (i.e. "two" side cannot have a valid filespec), so it
can never be a rename destination.

Use the DIFF_PAIR_UNMERGED() to detect unmerged filepair correctly, and
allow the valid "one" side of an unmerged filepair to be considered a
potential rename source, but never to be considered a rename destination.

Commit message and first two test cases by Junio, the rest by Martin.

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diffcore-rename.c

@@ -439,7 +439,7 @@ void diffcore_rename(struct diff_options *options)
 			else
 				locate_rename_dst(p->two, 1);
 		}
-		else if (!DIFF_FILE_VALID(p->two)) {
+		else if (!DIFF_PAIR_UNMERGED(p) && !DIFF_FILE_VALID(p->two)) {
 			/*
 			 * If the source is a broken "delete", and
 			 * they did not really want to get broken,
@@ -574,7 +574,10 @@ void diffcore_rename(struct diff_options *options)
 		struct diff_filepair *p = q->queue[i];
 		struct diff_filepair *pair_to_free = NULL;
 
-		if (!DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two)) {
+		if (DIFF_PAIR_UNMERGED(p)) {
+			diff_q(&outq, p);
+		}
+		else if (!DIFF_FILE_VALID(p->one) && DIFF_FILE_VALID(p->two)) {
 			/*
 			 * Creation
 			 *

t/t7060-wtstatus.sh

@@ -56,4 +56,66 @@ test_expect_success 'M/D conflict does not segfault' '
 	)
 '
 
+test_expect_success 'rename & unmerged setup' '
+	git rm -f -r . &&
+	cat "$TEST_DIRECTORY/README" >ONE &&
+	git add ONE &&
+	test_tick &&
+	git commit -m "One commit with ONE" &&
+
+	echo Modified >TWO &&
+	cat ONE >>TWO &&
+	cat ONE >>THREE &&
+	git add TWO THREE &&
+	sha1=$(git rev-parse :ONE) &&
+	git rm --cached ONE &&
+	(
+		echo "100644 $sha1 1	ONE" &&
+		echo "100644 $sha1 2	ONE" &&
+		echo "100644 $sha1 3	ONE"
+	) | git update-index --index-info &&
+	echo Further >>THREE
+'
+
+test_expect_success 'rename & unmerged status' '
+	git status -suno >actual &&
+	cat >expect <<-EOF &&
+	UU ONE
+	AM THREE
+	A  TWO
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'git diff-index --cached shows 2 added + 1 unmerged' '
+	cat >expected <<-EOF &&
+	U	ONE
+	A	THREE
+	A	TWO
+	EOF
+	git diff-index --cached --name-status HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'git diff-index --cached -M shows 2 added + 1 unmerged' '
+	cat >expected <<-EOF &&
+	U	ONE
+	A	THREE
+	A	TWO
+	EOF
+	git diff-index --cached --name-status HEAD >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success 'git diff-index --cached -C shows 2 copies + 1 unmerged' '
+	cat >expected <<-EOF &&
+	U	ONE
+	C	ONE	THREE
+	C	ONE	TWO
+	EOF
+	git diff-index --cached -C --name-status HEAD |
+	sed "s/^C[0-9]*/C/g" >actual &&
+	test_cmp expected actual
+'
+
 test_done