commit-tree: resurrect command line parsing updates

commit-tree: resurrect command line parsing updates

79a9312 (commit-tree: update the command line parsing, 2011-11-09)
updated the command line parser to understand the usual "flags first
and then non-flag arguments" order, in addition to the original and
a bit unusual "tree comes first and then zero or more -p <parent>".

Unfortunately, ba3c69a (commit: teach --gpg-sign option, 2011-10-05)
broke it by mistake.  Resurrect it, and protect the feature with a
test from future breakages.

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