commit-tree: add and document --no-gpg-sign

Document how to override commit.gpgsign configuration that is set to
true per "git commit" invocation (parse-options machinery lets us
say "--no-gpg-sign" to do so).

"git commit-tree" does not use parse-options, so manually add the
corresponding option for now.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit-tree.c

@@ -71,6 +71,11 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 			continue;
 		}
 
+		if (!strcmp(arg, "--no-gpg-sign")) {
+			sign_commit = NULL;
+			continue;
+		}
+
 		if (!strcmp(arg, "-m")) {
 			if (argc <= ++i)
 				usage(commit_tree_usage);