sequencer: run post-commit hook

sequencer: run post-commit hook

Prior to commit 356ee4659b ("sequencer: try to commit without forking
'git commit'", 2017-11-24) the sequencer would always run the
post-commit hook after each pick or revert as it forked `git commit` to
create the commit. The conversion to committing without forking `git
commit` omitted to call the post-commit hook after creating the commit.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>