http-push: widen `start_put()`'s size local from `ssize_t` to `size_t`

The local is initialised from `git_deflate_bound()` (an unsigned upper bound on the deflated output, never negative) and used in exactly three places: the initialising assignment, `strbuf_grow(buf, size)` whose parameter is already `size_t`, and `stream.avail_out` which became `size_t` in the prior commit. There is no comparison against zero or a negative value, no subtraction, no arithmetic that depends on signedness, and no path that would assign a signed quantity to it. The original `ssize_t` was the wrong type to begin with: a `git_deflate_bound()` result above `SSIZE_MAX` would have wrapped negative on assignment and then implicitly re-extended to a huge `size_t` at `strbuf_grow()`/`stream.avail_out`, requesting an absurd allocation. That is not a real-world concern for the object sizes http-push pushes today, but it is also the reason the type needs to move to `size_t` before `git_deflate_bound()` itself is widened. Assisted-by: Opus 4.7 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>