merge: split reduce_parents() out of collect_parents()

The latter does two separate things:

 - Parse the list of commits on the command line, and formulate the
   list of commits to be merged (including the current HEAD);

 - Compute the list of parents to be recorded in the resulting merge
   commit.

Split the latter into a separate helper function, so that we can
later supply the list commits to be merged from a different source
(namely, FETCH_HEAD).
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1044,23 +1044,11 @@ static int default_edit_option(void)
 		st_stdin.st_mode == st_stdout.st_mode);
 }
 
-static struct commit_list *collect_parents(struct commit *head_commit,
-					   int *head_subsumed,
-					   int argc, const char **argv)
+static struct commit_list *reduce_parents(struct commit *head_commit,
+					  int *head_subsumed,
+					  struct commit_list *remoteheads)
 {
-	int i;
-	struct commit_list *remoteheads = NULL, *parents, *next;
-	struct commit_list **remotes = &remoteheads;
-
-	if (head_commit)
-		remotes = &commit_list_insert(head_commit, remotes)->next;
-	for (i = 0; i < argc; i++) {
-		struct commit *commit = get_merge_parent(argv[i]);
-		if (!commit)
-			help_unknown_ref(argv[i], "merge",
-					 "not something we can merge");
-		remotes = &commit_list_insert(commit, remotes)->next;
-	}
+	struct commit_list *parents, *next, **remotes = &remoteheads;
 
 	/*
 	 * Is the current HEAD reachable from another commit being
@@ -1088,6 +1076,27 @@ static struct commit_list *collect_parents(struct commit *head_commit,
 	return remoteheads;
 }
 
+static struct commit_list *collect_parents(struct commit *head_commit,
+					   int *head_subsumed,
+					   int argc, const char **argv)
+{
+	int i;
+	struct commit_list *remoteheads = NULL;
+	struct commit_list **remotes = &remoteheads;
+
+	if (head_commit)
+		remotes = &commit_list_insert(head_commit, remotes)->next;
+	for (i = 0; i < argc; i++) {
+		struct commit *commit = get_merge_parent(argv[i]);
+		if (!commit)
+			help_unknown_ref(argv[i], "merge",
+					 "not something we can merge");
+		remotes = &commit_list_insert(commit, remotes)->next;
+	}
+
+	return reduce_parents(head_commit, head_subsumed, remoteheads);
+}
+
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
 	unsigned char result_tree[20];