commit-tree: document -S option consistently
Commit
ba3c69a9 (commit: teach --gpg-sign option, 2011-10-05) added the
-S option but documented it in the command usage without indicating that
the value is optional and forgot to mention it in the manpage. Later
commit
098bbdc3 (Add -S, --gpg-sign option to manpage of "git commit",
2012-10-21) documented the option in the porcelain manpage.
Use wording from the porcelain manpage to document the option in the
plumbing manpage. Also update the commit-tree usage summary to indicate
that the -S value is optional to be consistent with the manpage and with
the implementation.
Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>