Merge branch 'mb/filter-branch-optim'

Merge branch 'mb/filter-branch-optim'

"git filter-branch" when used with the "--state-branch" option
still attempted to rewrite the commits whose filtered result is
known from the previous attempt (which is recorded on the state
branch); the command has been corrected not to waste cycles doing
so.

* mb/filter-branch-optim:
  filter-branch: skip commits present on --state-branch