Fix check_file_directory_conflict().

When replacing an existing file A with a directory A that has a
file A/B in it in the index, 'update-index --replace --add A/B'
did not properly remove the file to make room for the new
directory.

There was a trivial logic error, most likely a cut & paste one,
dating back to quite early days of git.

Signed-off-by: Junio C Hamano <junkio@cox.net>

read-cache.c

diff --git a/read-cache.c b/read-cache.c
index 983e68b..b8d83cc 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -517,7 +517,7 @@
 		pos = cache_name_pos(name, ntohs(create_ce_flags(len, stage)));
 		if (pos >= 0) {
 			retval = -1;
-			if (ok_to_replace)
+			if (!ok_to_replace)
 				break;
 			remove_cache_entry_at(pos);
 			continue;

t/t0000-basic.sh

diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
index 3260d1d..0cd1c41 100755
--- a/t/t0000-basic.sh
+++ b/t/t0000-basic.sh
@@ -272,4 +272,13 @@
 	 wc -l) &&
      test $numparent = 1'
 
+test_expect_success 'update-index D/F conflict' '
+	mv path0 tmp &&
+	mv path2 path0 &&
+	mv tmp path2 &&
+	git update-index --add --replace path2 path0/file2 &&
+	numpath0=$(git ls-files path0 | wc -l) &&
+	test $numpath0 = 1
+'
+
 test_done