diffcore-rename: fall back to -C when -C -C busts the rename limit

When there are too many paths in the project, the number of rename source
candidates "git diff -C -C" finds will exceed the rename detection limit,
and no inexact rename detection is performed.  We however could fall back
to "git diff -C" if the number of modified paths is sufficiently small.

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

t/t4001-diff-rename.sh

@@ -77,4 +77,29 @@ test_expect_success  'favour same basenames even with minor differences' '
 	git show HEAD:path1 | sed "s/15/16/" > subdir/path1 &&
 	git status | grep "renamed: .*path1 -> subdir/path1"'
 
+test_expect_success 'setup for many rename source candidates' '
+	git reset --hard &&
+	for i in 0 1 2 3 4 5 6 7 8 9;
+	do
+		for j in 0 1 2 3 4 5 6 7 8 9;
+		do
+			echo "$i$j" >"path$i$j"
+		done
+	done &&
+	git add "path??" &&
+	test_tick &&
+	git commit -m "hundred" &&
+	(cat path1; echo new) >new-path &&
+	echo old >>path1 &&
+	git add new-path path1 &&
+	git diff -l 4 -C -C --cached --name-status >actual 2>actual.err &&
+	sed -e "s/^\([CM]\)[0-9]*	/\1	/" actual >actual.munged &&
+	cat >expect <<-EOF &&
+	C	path1	new-path
+	M	path1
+	EOF
+	test_cmp expect actual.munged &&
+	grep warning actual.err
+'
+
 test_done