handle revision==None. fixes #262.
handle revision==None. fixes #262.

--HG--
extra : convert_revision : svn%3A3ed01bd8-26fb-0310-9e4c-ca1a4053419f/networkx/trunk%401401

JamesClough/networkx,dmoliveira/networkx,kernc/networkx,farhaanbukhsh/networkx,harlowja/networkx,OrkoHunter/networkx,dmoliveira/networkx,ionanrozenfeld/networkx,michaelpacer/networkx,debsankha/networkx,ghdk/networkx,nathania/networkx,kernc/networkx,blublud/networkx,aureooms/networkx,RMKD/networkx,bzero/networkx,SanketDG/networkx,bzero/networkx,farhaanbukhsh/networkx,wasade/networkx,harlowja/networkx,blublud/networkx,jni/networkx,farhaanbukhsh/networkx,andnovar/networkx,NvanAdrichem/networkx,dmoliveira/networkx,kai5263499/networkx,debsankha/networkx,dhimmel/networkx,jni/networkx,bzero/networkx,jtorrents/networkx,chrisnatali/networkx,beni55/networkx,jtorrents/networkx,blublud/networkx,ltiao/networkx,chrisnatali/networkx,RMKD/networkx,goulu/networkx,nathania/networkx,sharifulgeo/networkx,ionanrozenfeld/networkx,kai5263499/networkx,kai5263499/networkx,tmilicic/networkx,aureooms/networkx,RMKD/networkx,nathania/networkx,jakevdp/networkx,cmtm/networkx,ghdk/networkx,kernc/networkx,harlowja/networkx,jcurbelo/networkx,sharifulgeo/networkx,jni/networkx,Sixshaman/networkx,dhimmel/networkx,jakevdp/networkx,ionanrozenfeld/networkx,ghdk/networkx,dhimmel/networkx,jakevdp/networkx,chrisnatali/networkx,jfinkels/networkx,yashu-seth/networkx,aureooms/networkx,debsankha/networkx,sharifulgeo/networkx