merge: fix crash on criss cross merge with dir move and delete (issue5020)
Work around that 'dm' in the data model only can have one operation for the
target file, but still can have multiple and conflicting operations on the
source file where the other operation is a 'rm'. The move would thus fail with
'abort: No such file or directory'.
In this case it is "obvious" that the file should be removed, either before or
after moving it. We thus keep the 'rm' of the source file but drop the 'dm'.
This is not a pretty fix but quite "obviously" safe (famous last words...) as
it only touches a rare code path that used to crash. It is possible that it
would be better to swap the files for 'dm' as suggested on
https://bz.mercurial-scm.org/show_bug.cgi?id=5020#c13 but it is not entirely
obvious that it not just would create conflicts on the other file. That can be