rebase -i: Enclose sed command substitution in quotes

Reported by: Johannes Sixt <j.sixt@viscovery.net>

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/lib-rebase.sh

@@ -27,7 +27,7 @@ set_fake_editor () {
 case "$1" in
 */COMMIT_EDITMSG)
 	test -z "$EXPECT_HEADER_COUNT" ||
-		test "$EXPECT_HEADER_COUNT" = $(sed -n '1s/^# This is a combination of \(.*\) commits\./\1/p' < "$1") ||
+		test "$EXPECT_HEADER_COUNT" = "$(sed -n '1s/^# This is a combination of \(.*\) commits\./\1/p' < "$1")" ||
 		exit
 	test -z "$FAKE_COMMIT_MESSAGE" || echo "$FAKE_COMMIT_MESSAGE" > "$1"
 	test -z "$FAKE_COMMIT_AMEND" || echo "$FAKE_COMMIT_AMEND" >> "$1"