Merge branch 'jk/rebase-i-exec-gitdir-fix' into maint

A recent regression in "git rebase -i" that broke execution of git
commands from subdirectories via "exec" insn has been fixed.

* jk/rebase-i-exec-gitdir-fix:
  sequencer: pass absolute GIT_DIR to exec commands

diff --cc sequencer.c
Simple merge

diff --cc t/t3404-rebase-interactive.sh
Simple merge