filemerge: add __bytes__ for absentfilectx

This will at _least_ aid some upcoming debugging.

Differential Revision: https://phab.mercurial-scm.org/D8592

mercurial/filemerge.py

@@ -98,6 +98,9 b' class absentfilectx(object):'
         self._ctx = ctx
         self._f = f
 
+    def __bytes__(self):
+        return b'absent file %s@%s' % (self._f, self._ctx)
+
     def path(self):
         return self._f