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: NJunio C Hamano <gitster@pobox.com>

Documentation/git-commit-tree.txt

@@ -55,8 +55,13 @@ OPTIONS
 	from the standard input.
 
 -S[<keyid>]::
+--gpg-sign[=<keyid>]::
 	GPG-sign commit.
 
+--no-gpg-sign::
+	Countermand `commit.gpgsign` configuration variable that is
+	set to force each and every commit to be signed.
+
 
 Commit Information
 ------------------

Documentation/git-commit.txt

@@ -302,6 +302,10 @@ configuration variable documented in linkgit:git-config[1].
 --gpg-sign[=<keyid>]::
 	GPG-sign commit.
 
+--no-gpg-sign::
+	Countermand `commit.gpgsign` configuration variable that is
+	set to force each and every commit to be signed.
+
 \--::
 	Do not interpret any more arguments as options.
 

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);