sequencer: refactor the code to obtain a short commit name

sequencer: refactor the code to obtain a short commit name

Not only does this DRY up the code (providing a better documentation what
the code is about, as well as allowing to change the behavior in a single
place), it also makes it substantially shorter to use the same
functionality in functions to be introduced when we teach the sequencer to
process interactive-rebase's git-rebase-todo file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>