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

contrib/completion/git-completion.bash

@@ -1037,6 +1037,7 @@ _git_merge ()
 	--*)
 		__gitcomp "
 			--no-commit --no-stat --log --no-log --squash --strategy
+			--commit --stat --no-squash --ff --no-ff
 			"
 		return
 	esac