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.
adding a
adding b
a
NONEXISTENT: No such file or directory
a
b
a: No such file or directory
NONEXISTENT: No such file or directory
b
a
NONEXISTENT: No such file in rev 14467d15ef43
a
b