Move WebDAV HTTP push under remote-curl

Move WebDAV HTTP push under remote-curl

The remote helper interface now supports the push capability,
which can be used to ask the implementation to push one or more
specs to the remote repository.  For remote-curl we implement this
by calling the existing WebDAV based git-http-push executable.

Internally the helper interface uses the push_refs transport hook
so that the complexity of the refspec parsing and matching can be
reused between remote implementations.  When possible however the
helper protocol uses source ref name rather than the source SHA-1,
thereby allowing the helper to access this name if it is useful.

>From Clemens Buchacher <drizzd@aon.at>:
 update http tests according to remote-curl capabilities

 o Pushing packed refs is now fixed.

 o The transport helper fails if refs are already up-to-date. Add
   a test for that.

 o The transport helper will notice if refs are already
   up-to-date. We therefore need to update server info in the
   unpacked-refs test.

 o The transport helper will purge deleted branches automatically.

 o Use a variable ($ORIG_HEAD) instead of full SHA-1 name.

Signed-off-by: Tay Ray Chuan <rctay89@gmail.com>
Signed-off-by: Clemens Buchacher <drizzd@aon.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
CC: Daniel Barkalow <barkalow@iabervon.org>
CC: Mike Hommey <mh@glandium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>