strvec: use size_t to store nr and alloc

strvec: use size_t to store nr and alloc

We converted argv_array (which later became strvec) to use size_t in
819f0e76b1 (argv-array: use size_t for count and alloc, 2020-07-28) in
order to avoid the possibility of integer overflow. But later, commit
d70a9eb611 (strvec: rename struct fields, 2020-07-28) accidentally
converted these back to ints!

Those two commits were part of the same patch series. I'm pretty sure
what happened is that they were originally written in the opposite order
and then cleaned up and re-ordered during an interactive rebase. And
when resolving the inevitable conflict, I mistakenly took the "rename"
patch completely, accidentally dropping the type change.

We can correct it now; better late than never.

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