Fix merge-tools.checkconflicts

re.match only looks at the beginning of the merged file, and without
re.MULTILINE the file had to end with ">>>>>>> something".

Now conflict markers inside the file are found, too.

mercurial/filemerge.py

@@ -225,7 +225,8 b' def filemerge(repo, mynode, orig, fcd, f'
 
     if not r and (_toolbool(ui, tool, "checkconflicts") or
                   'conflicts' in _toollist(ui, tool, "check")):
-        if re.match("^(<<<<<<< .*|=======|>>>>>>> .*)$", fcd.data()):
+        if re.search("^(<<<<<<< .*|=======|>>>>>>> .*)$", fcd.data(),
+                     re.MULTILINE):
             r = 1
 
     checked = False