receive-pack: add another option for receive.denyCurrentBranch

When synchronizing between working directories, it can be handy to update
the current branch via 'push' rather than 'pull', e.g. when pushing a fix
from inside a VM, or when pushing a fix made on a user's machine (where
the developer is not at liberty to install an ssh daemon let alone know
the user's password).

The common workaround – pushing into a temporary branch and then merging
on the other machine – is no longer necessary with this patch.

The new option is:

'updateInstead':
	Update the working tree accordingly, but refuse to do so if there
	are any uncommitted changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2129,6 +2129,13 @@ receive.denyCurrentBranch::
 	print a warning of such a push to stderr, but allow the push to
 	proceed. If set to false or "ignore", allow such pushes with no
 	message. Defaults to "refuse".
++
+Another option is "updateInstead" which will update the working
+directory (must be clean) if pushing into the current branch. This option is
+intended for synchronizing working directories when one side is not easily
+accessible via interactive ssh (e.g. a live web site, hence the requirement
+that the working directory be clean). This mode also comes in handy when
+developing inside a VM to test and fix code on different Operating Systems.
 
 receive.denyNonFastForwards::
 	If set to true, git-receive-pack will deny a ref update which is