builtin-push: add --delete as syntactic sugar for :foo

builtin-push: add --delete as syntactic sugar for :foo

Refspecs without a source side have been reported as confusing by many.
As an alternative, this adds support for commands like:

    git push origin --delete somebranch
    git push origin --delete tag sometag

Specifically, --delete will prepend a colon to all colon-less refspecs
given on the command line, and will refuse to accept refspecs with
colons to prevent undue confusion.

Signed-off-by: Jan Krüger <jk@jk.gs>
Signed-off-by: Junio C Hamano <gitster@pobox.com>