sequencer: simplify root commit creation

sequencer: simplify root commit creation

Adapt try_to_commit() to create a new root commit rather than special
casing this in run_git_commit(). This significantly reduces the amount of
special case code for creating the root commit and reduces the number of
commit code paths we have to worry about.

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