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>

http-push.c

@@ -367,7 +367,7 @@ static void start_put(struct transfer_request *request)
 	void *unpacked;
 	size_t len;
 	int hdrlen;
-	ssize_t size;
+	size_t size;
 	git_zstream stream;
 	struct repo_config_values *cfg = repo_config_values(the_repository);