merge: use help_unknown_ref()

Use help.c:help_unknown_ref() instead of die() to provide a
friendlier error message before exiting, when one of the refs
specified in a merge is unknown.

Signed-off-by: Vikrant Varma <vikrant.varma94@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1054,7 +1054,8 @@ static struct commit_list *collect_parents(struct commit *head_commit,
 	for (i = 0; i < argc; i++) {
 		struct commit *commit = get_merge_parent(argv[i]);
 		if (!commit)
-			die(_("%s - not something we can merge"), argv[i]);
+			help_unknown_ref(argv[i], "merge",
+					 "not something we can merge");
 		remotes = &commit_list_insert(commit, remotes)->next;
 	}
 	*remotes = NULL;