git_connect: use argv_array

git_connect: use argv_array

This avoids magic numbers when we allocate fixed-size argv
arrays, and makes it more obvious that we are not
overflowing.

It is also the first step to fixing a memory leak. When
git_connect returns a child_process struct, the argv array
in the struct is dynamically allocated, but the individual
strings are not (they are either owned elsewhere, or are
freed). Later, in finish_connect, we free the array but
leave the strings alone.

This works for the child_process created by git_connect, but
if we use transport_take_over, we may also end up with a
child_process created by transport-helper's get_helper.
In that case, the strings are freshly allocated, and we
would want to free them.  However, we have no idea in
finish_connect which type we have.

By consistently using run-command's internal argv-array, we
do not have to worry about this issue at all; finish_command
takes care of it for us, and we can drop our manual free
entirely.

Note that this actually makes the get_helper leak slightly
worse; now we are leaking both the strings and the array.
But when we adjust it in a future patch, that leak will go
away entirely.

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