filemerge: when merge tool uses $output, don't leave markers in $local

As explained in the previous patch, we incorrectly leave conflict
markers in both `$local` and `$output` since D12190. I don't
understand why it broke but the fix is simple and clear after all the
recent refactoring.

Differential Revision: https://phab.mercurial-scm.org/D12379

mercurial/filemerge.py

@@ -755,8 +755,7 b' def _xmerge(repo, mynode, local, other, '
         # Remove the .orig to make syntax-highlighting more likely.
         if localoutputpath.endswith(b'.orig'):
             localoutputpath, ext = os.path.splitext(localoutputpath)
-        localdata = util.readfile(localpath)
-        files.append((b"local", localoutputpath, localdata))
+        files.append((b"local", localoutputpath, backup.data()))
 
     with _maketempfiles(files) as temppaths:
         basepath, otherpath = temppaths[:2]

tests/test-merge-tools.t

@@ -1225,12 +1225,8 b' with premerge=keep and $output in tool a'
   merging f
   revision 0
   space
-  <<<<<<< working copy: ef83787e2614 - test: revision 1 (known-bad-output !)
   revision 1
   space
-  ======= (known-bad-output !)
-  revision 4 (known-bad-output !)
-  >>>>>>> merge rev:    81448d39c9a0 - test: revision 4 (known-bad-output !)
   revision 4
   <<<<<<< working copy: ef83787e2614 - test: revision 1
   revision 1