Patch to make 'hg checkout <TAB>' complete bookmark names in addition to
tag names.

This patch also modifies the parsing of 'hg tags' in the same context in
two ways.  It employs --quiet to simplify the hg output being parsed,
and stops using _describe, which makes tag names containing backslashes
complete correctly.  (The problem is that _describe parses colons and
backslashes within its arguments, and hg tag names were passed to
_describe without escaping.  Only backslashes may trigger the problem
because hg does not permit tag names to have colons.)

Cheers,

Daniel
(In the above, 'tag names' means hg tags, not zshcompsys tags)