git-am: Add colon before the subject that is printed out as being applied

git-am output can be confusing, because the subject of the applied
patch can look like the rest of a sentence starting with "Applying".
The added colon should make this clearer.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -456,7 +456,7 @@ do
 		stop_here $this
 	fi
 
-	printf 'Applying %s\n' "$FIRSTLINE"
+	printf 'Applying: %s\n' "$FIRSTLINE"
 
 	case "$resolved" in
 	'')

t/t4151-am-abort.sh

@@ -43,7 +43,7 @@ do
 
 	test_expect_success "am$with3 --skip continue after failed am$with3" '
 		test_must_fail git-am$with3 --skip >output &&
-		test "$(grep "^Applying" output)" = "Applying 6" &&
+		test "$(grep "^Applying" output)" = "Applying: 6" &&
 		test_cmp file-2-expect file-2 &&
 		test ! -f .git/rr-cache/MERGE_RR
 	'