run-command: add "use shell" option

run-command: add "use shell" option

Many callsites run "sh -c $CMD" to run $CMD. We can make it
a little simpler for them by factoring out the munging of
argv.

For simple cases with no arguments, this doesn't help much, but:

  1. For cases with arguments, we save the caller from
     having to build the appropriate shell snippet.

  2. We can later optimize to avoid the shell when
     there are no metacharacters in the program.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>