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>