sequencer: make commit options more extensible

sequencer: make commit options more extensible

So far every time we need to tweak the behaviour of run_git_commit()
we have been adding a "int" parameter to it.  As the function gains
parameters and different callsites having different needs, this is
becoming a maintenance burden.  When a new knob needs to be added to
address a specific need for a single callsite, all the other callsites
need to add a "no, I do not want anything special with respect to the
new knob" argument.

Consolidate the existing four parameters into a flag word to make it
more maintainable, as we will be adding a new one to the mix soon.

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