"git rebase" started exporting GIT_DIR environment variable and
exposing it to hook scripts when part of it got rewritten in C.
Instead of matching the old scripted Porcelains' behaviour,
compensate by also exporting GIT_WORK_TREE environment as well to
lessen the damage.  This can harm existing hooks that want to
operate on different repository, but the current behaviour is
already broken for them anyway.

* bc/sequencer-export-work-tree-as-well:
  sequencer: pass absolute GIT_WORK_TREE to exec commands