execv_dashed_external: stop exiting with negative code

execv_dashed_external: stop exiting with negative code

When we try to exec a git sub-command, we pass along the
status code from run_command(). But that may return -1 if we
ran into an error with pipe() or execve(). This tends to
work (and end up as 255 due to twos-complement wraparound
and truncation), but in general it's probably a good idea to
avoid negative exit codes for portability.

We can easily translate to the normal generic "128" code we
get when syscalls cause us to die.

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