revlog: add a ways to blacklist some revision when searching for a delta
This will be useful to recompute appropriate deltas one the fly during
censor/strip operation with revlog-v2.
Differential Revision: https://phab.mercurial-scm.org/D10799
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