filemerge: drop a default argument to appease pytype

The function slices and takes the length of this argument without internally
setting it if not provided.  There was no bug here because both callers passed
the argument.

Differential Revision: https://phab.mercurial-scm.org/D7464

mercurial/filemerge.py

@@ -693,7 +693,7 b' def _describemerge(ui, repo, mynode, fcl'
     ui.status(t.renderdefault(props))
 
 
-def _xmerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels=None):
+def _xmerge(repo, mynode, orig, fcd, fco, fca, toolconf, files, labels):
     tool, toolpath, binary, symlink, scriptfn = toolconf
     uipathfn = scmutil.getuipathfn(repo)
     if fcd.isabsent() or fco.isabsent():