bash: add missing 'git merge' options

bash: add missing 'git merge' options

Namely: '--commit', '--stat', '--no-squash', '--ff', '--no-ff'.

One might wonder why add options that specify the default behaviour
anyway (e.g. '--commit', '--no-squash', etc.).  Users can override the
default with config options (e.g. 'branch.<name>.mergeoptions',
'merge.log'), but sometimes might still need the default behaviour.

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>