Add a few more values for receive.denyCurrentBranch

Add a few more values for receive.denyCurrentBranch

For a long time, this developer thought that Git's insistence that
pushing into the current branch is evil was completely merited.

Just for fun, the original patch tried to show people that Git is
correct to forbid that, and that it causes more trouble than it does
good when Git allows you to try to update the working tree for
fast-forwards, or to detach the HEAD, depending on some config settings.

To the developer's surprise, the opposite was shown.

So here is the support for two new options you can give the config
variable receive.denyCurrentBranch:

'updateInstead':
	Try to merge the working tree with the new tip of the branch
	(which can lead to really horrible merge conflicts).

'detachInstead':
	Detach the HEAD, thereby avoiding a disagreement between the
	HEAD and the index (as well as the working tree), possibly
	leaving the local user wondering how on earth her HEAD became
	so detached.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>