doc/README

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
false positive.

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

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