new extension: fetch -> combine pull and merge/update
pull changes from remote repo.
if changes add new head: merge with new head, commit result.
else: update working dir to new head.
this makes useful command like git 'pull' command.
#header#
# HG changeset patch
# User #author#
# Date #date|hgdate#
# Node ID #node#
#parent%changesetparent#
#desc#
#diff#