SCRIPTS: make announce release support preparing announces before tag exists

SCRIPTS: make announce release support preparing announces before tag exists

It takes so much time to write an announce message that sometimes it's
annoying not being able to start the work while a fix is being finished.
With the new "-p" argument, announce-release will allow to prepare the
announce message for the current HEAD and with no tag yet. It will
restart from the last tag and automatically increment the version using
the same algorithm as create-release so that everything is accurate. It
should then be easier at the last moment to just include the final entry
by hand when the last fix finally arrives. For convenience, this argument
also allows to create an announce from another branch than master.