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.

changeset:   0:9426b370c206
tag:         tip
user:        My Name <myname@example.com>
date:        Mon Jan 12 13:46:40 1970 +0000
summary:     commit-1