Don't specify a revision to pull from a mercurial repository

Buildbot trac ticket 438.  Pulling a tag does not guarantee
that mercurial can update to that tag.
Merge branch 'mercurial_tag' of git://github.com/pepsiman/buildbot
Merge branch 'reason' of git://github.com/pepsiman/buildbot
Fix reason for Triggerable scheduler.  Add reason tests