graphlog: fix --removed
"hg log --removed FILE" does not return changesets where FILE was removed, but
ones where FILE was changed and possibly removed. The flag is really here to
disable walkchangerevs() fast path, which cannot see file removals by scanning
filelogs.
Greg Ward, author of the original bfiles extension
Na'Tosha Bard of Unity Technologies
Fog Creek Software
Special thanks to the University of Toronto and the UCOSP program