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.

--HG--
branch : stable
extra : source : 44d7bfe08c14bd389d18b8bfd3d5687809ad3f62
histedit: add more detailed help about "--outgoing"

--HG--
branch : stable
extra : source : e7c0e5f81367b386befc2ed6714933113d05b307