revert: fix corner case found by faheem mitha.
if file not in dest manifest, remove only if in source manifest.
Fix
issue240: mq: qpush fails on Solaris
Force editor to appear for an 'hg backout' if a message or logfile aren't
supplied. Pre-fill the commit message with the changeset being backed out.
Fixes bug 238.
hooks: add preupdate and update hooks.
preupdate run before working dir updated. can prevent update.
update run after working dir updated.