completion: add missing options for git-merge

The options added to __git_merge_options are those that git-pull passes
to git-merge, since that variable is used by both commands.

Those added directly in _git_merge() are specific to git-merge and
are not passed thru from git-pull.
Reported-by: NHaralan Dobrev <hkdobrev@gmail.com>
Signed-off-by: NJohn Keeping <john@keeping.me.uk>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1476,6 +1476,8 @@ _git_log ()
 __git_merge_options="
 	--no-commit --no-stat --log --no-log --squash --strategy
 	--commit --stat --no-squash --ff --no-ff --ff-only --edit --no-edit
+	--verify-signatures --no-verify-signatures --gpg-sign
+	--quiet --verbose --progress --no-progress
 "
 
 _git_merge ()
@@ -1484,7 +1486,8 @@ _git_merge ()
 
 	case "$cur" in
 	--*)
-		__gitcomp "$__git_merge_options"
+		__gitcomp "$__git_merge_options
+			--rerere-autoupdate --no-rerere-autoupdate --abort"
 		return
 	esac
 	__gitcomp_nl "$(__git_refs)"