Bug in bash completion for git rebase --onto

Hi,

The bash completion for git rebase --onto seems to have been broken by commit 2b9bd488ae09dc89f02be7d73f3710ee2ea7325c

Previously (v2.23.0), hitting tab after

    git rebase --on

gives:

    git rebase --onto

(note trailing space) hitting tab again offers a list of potential refs onto which to rebase.

As of 2b9bd488ae09dc89f02be7d73f3710ee2ea7325c, hitting tab after:

    git rebase --on

gives:

    git rebase --onto=

but then hitting tab again offers the current directory contents as candidates, i.e. not git ref candidates as they should be.

Note the adding of a trailing '=' is, I think, more correct/robust than the trailing space so I don't disagree with that change. It's just the failure to offer git ref candidates that is the problem.

Thanks,

Paul