histedit: abort if there are multiple roots in "--outgoing" revisions
Before this patch, if there are multiple roots in "--outgoing"
revisions, result of "histedit --outgoing" depends on the parent of
the working directory. It succeeds only when the parent of the working
directory is a descendant of the oldest root in "--outgoing"
revisions, and fails otherwise.
It seems to be ambiguous and difficult for users.
This patch makes "histedit --outgoing" abort if there are multiple
roots in "--outgoing" revisions always.