branch --merged/--no-merged: allow specifying arbitrary commit

branch --merged/--no-merged: allow specifying arbitrary commit

"git-branch --merged" is a handy way to list all the branches that have
already been merged to the current branch, but it did not allow checking
against anything but the current branch.  Having to switch branches only
to list the branches that are merged with another branch made the feature
practically useless.

This updates the option parser so that "git branch --merged next" is
accepted when you are on 'master' branch.

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