zsh_completion: use revsets to exclude this rev from suggestions to hg merge

Authored by av6.

Description

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
output.

While at it, declare branches as an array too.

Differential Revision: https://phab.mercurial-scm.org/D4426