zsh_completion: use revsets to exclude this rev from suggestions to hg merge
One of the most important aspects of a completion system is its speed, so 1
call to hg is definitely better than 4.
Sorting by rev (descending) is to preserve the same order as in hg heads
While at it, declare branches as an array too.
Differential Revision: https://phab.mercurial-scm.org/D4426