parse-options: ambiguous LASTARG_DEFAULT and OPTARG

Hi,

This in builtin-branch.c

        {
		OPTION_CALLBACK, 0, "merged", &merge_filter_ref,
		"commit", "print only merged branches",
		PARSE_OPT_LASTARG_DEFAULT | PARSE_OPT_NONEG,
		opt_parse_merge_filter, (intptr_t) "HEAD",
	},

and the usage message for "git-branch -h" will print out

    --merged <commit>

when I'm expecting

    --merged[=<commit>]

This is because the PARSE_OPT_OPTARG flag is not used. Is this correct?
The default value is still set correctly in some cases, but become
ambiguous in other cases. Take this for example

    $ git branch --merged --verbose
    fatal: malformed object name --verbose

but

    $ git branch --verbose --merged

works fine.

The simple fix is to just add PARSE_OPT_OPTARG to the flags, and fix a
test or two. But I'm wondering if doing that will become problematic for
end-users. Essentially you can no longer do git branch --merged master,
you must do git branch --merged=master.