contrib: subtree: adjust test to change in fmt-merge-msg

We're starting to stop treating `master' specially in fmt-merge-msg.
Adjust the test to reflect that change.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/subtree/t/t7900-subtree.sh

@@ -196,7 +196,8 @@ test_expect_success 'merge new subproj history into sub dir/ with --prefix' '
 		cd "$subtree_test_count" &&
 		git fetch ./"sub proj" master &&
 		git subtree merge --prefix="sub dir" FETCH_HEAD &&
-		check_equal "$(last_commit_message)" "Merge commit '\''$(git rev-parse FETCH_HEAD)'\''"
+		check_equal "$(last_commit_message)" \
+			"Merge commit '\''$(git rev-parse FETCH_HEAD)'\'' into master"
 	)
 '
 
@@ -273,7 +274,8 @@ test_expect_success 'merge new subproj history into subdir/ with a slash appende
 		cd "$test_count" &&
 		git fetch ./subproj master &&
 		git subtree merge --prefix=subdir/ FETCH_HEAD &&
-		check_equal "$(last_commit_message)" "Merge commit '\''$(git rev-parse FETCH_HEAD)'\''"
+		check_equal "$(last_commit_message)" \
+			"Merge commit '\''$(git rev-parse FETCH_HEAD)'\'' into master"
 	)
 '