bash: update 'git commit' completion

bash: update 'git commit' completion

I just wanted to add the recently learnt '--reset-author' option, but
then noticed that there are many more options missing.  This patch
adds support for all of 'git commit's options, except '--allow-empty',
because it is primarily there for foreign scm interfaces.

Furthermore, this patch also adds support for completing the arguments
of those options that take a non-filename argument: valid modes are
offered for '--cleanup' and '--untracked-files', while refs for
'--reuse-message' and '--reedit-message', because these two take a
commit as argument.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Acked-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>