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