context: delete mistaken comment about return value of renamed()

The comment seems to think that the return value of renamed() is a
tuple of source pathsx in the parents, but it's actually a pair of
(rename source path, file nodeid).

Differential Revision: https://phab.mercurial-scm.org/D5639

mercurial/context.py

@@ -2178,8 +2178,6 b' def memfilefromctx(ctx):'
     """
     def getfilectx(repo, memctx, path):
         fctx = ctx[path]
-        # this is weird but apparently we only keep track of one parent
-        # (why not only store that instead of a tuple?)
         copied = fctx.renamed()
         if copied:
             copied = copied[0]