fix GIT_TRACE segfault with shell-quoted aliases

fix GIT_TRACE segfault with shell-quoted aliases

The alias argv comes from the split_cmdline function, which
splits the config text for the alias into an array of
strings. It returns the number of elements in the array, but
does not actually put a NULL at the end of the array.
Later, the trace function tries to print this argv and
assumes that it has the trailing NULL.

The split_cmdline function is probably at fault, since argv
lists almost always end with a NULL signal. This patch adds
one, in addition to the returned count; this doesn't hurt
the other callers at all, since they were presumably using
the count already (and will never look at the NULL).

While we're there and using ALLOC_GROW, let's clean up the
other manual grow.

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