sequencer: use run_command() directly

sequencer: use run_command() directly

Instead of using the convenience function run_command_v_opt_cd_env(), we
now use the run_command() function. The former function is simply a
wrapper of the latter, trying to make it more convenient to use.

However, we already have to construct the argv and the env parameters,
and we will need even finer control e.g. over the output of the command,
so let's just stop using the convenience function.

Based on patches and suggestions by Johannes Sixt and Jeff King.

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