http-push: add support for deleting remote branches

Processes new command-line arguments -d and -D to remove a remote branch
if the following conditions are met:
- one branch name is present on the command line
- the specified branch name matches exactly one remote branch name
- the remote HEAD is a symref
- the specified branch is not the remote HEAD
- the remote HEAD resolves to an object that exists locally (-d only)
- the specified branch resolves to an object that exists locally (-d only)
- the specified branch is an ancestor of the remote HEAD (-d only)

Signed-off-by: Nick Hengeveld <nickh@reactrix.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>