[PATCH 3 of 3 STABLE] rust: add comment about lack of wdirrev handling

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1547953056 -32400
#      Sun Jan 20 11:57:36 2019 +0900
# Branch stable
# Node ID 13c23396c7fe1633a2336b29e3a32b9b76274f28
# Parent  66102f6fa10aa533a658532b198cd6879a538406
rust: add comment about lack of wdirrev handling

If hg is compiled with rust support, 'only(wdir())' crashed as
"rustext.GraphError: ('ParentOutOfRange', 2147483647)", which should instead
say "abort: working directory revision cannot be specified."

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -896,6 +896,8 @@ class revlog(object):
             common = [nullrev]
 
         if rustext is not None:
+            # TODO: WdirUnsupported should be raised instead of GraphError
+            # if common includes wdirrev
             return rustext.ancestor.MissingAncestors(self.index, common)
         return ancestor.incrementalmissingancestors(self.parentrevs, common)