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

mercurial/filemerge.py

@@ -1212,9 +1212,13 b' def _onfilemergefailure(ui):'
 
 
 def hasconflictmarkers(data):
+    # Detect lines starting with a string of 7 identical characters from the
+    # subset Mercurial uses for conflict markers, followed by either the end of
+    # line or a space and some text. Note that using [<>=+|-]{7} would detect
+    # `<><><><><` as a conflict marker, which we don't want.
     return bool(
         re.search(
-            br"^(<<<<<<<.*|=======.*|------- .*|\+\+\+\+\+\+\+ .*|>>>>>>>.*)$",
+            br"^([<>=+|-])\1{6}( .*)$",
             data,
             re.MULTILINE,
         )