sequencer: add a new function to silence a command, except if it fails

sequencer: add a new function to silence a command, except if it fails

This adds a new function, run_command_silent_on_success(), to
redirect the stdout and stderr of a command to a strbuf, and then to run
that command. This strbuf is printed only if the command fails. It is
functionnaly similar to output() from git-rebase.sh.

run_git_commit() is then refactored to use of
run_command_silent_on_success().

Signed-off-by: Alban Gruin <alban.gruin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>