filemerge: be more strict when detecting conflict markers, add | markers
I received a user complaint about detecting a line that contained 78 =
followed by */ as a conflict marker. We'll never generate that, we generate 7
identical characters and either the end of the line, or a space. Let's be
explicit about detecting exactly what we produce to reduce the chances of a
While we're here, add ||||||| as a detected conflict marker (generated with
the keep-merge3 style conflicts).
Differential Revision: https://phab.mercurial-scm.org/D11391