log: fix --follow null parent not to include revision 0 If p1 is null, ':.' is translated as '0:null'. But rangeset can't handle null, only revision 0 was visible. Because 'null' should not be listed implicitly, "log --follow" (without -r) should be empty if p1 is null. Test of "hg grep -f" is added for cmdutil.walkchangerevs().