git.c: avoid allocating one-too-many elements for new argv array

git.c: avoid allocating one-too-many elements for new argv array

When creating a new argv array from a configured alias and the supplied
command line arguments, the new argv was allocated with one element too
many.  Since the first element of the original argv array is skipped when
copying it to the new_argv, the number of elements that are allocated
should be reduced by one.  'count' is the number of elements that new_argv
contains, and *argcp is the number of elements in the original argv array.
So the total allocation (including the terminating NULL entry) for the
new_argv array should be:

  count + (*argcp - 1) + 1

Also, the explicit assignment of the NULL terminating entry can be avoided
by just copying it over from the original argv array.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>