Merge branch 'jk/filter-branch-no-index'

A recent optimization to filter-branch in v2.7.0 introduced a
regression when --prune-empty filter is used, which has been
corrected.

* jk/filter-branch-no-index:
  filter-branch: resolve $commit^{tree} in no-index case

diff --cc git-filter-branch.sh
Simple merge

diff --cc t/t7003-filter-branch.sh
Simple merge