http: use strbufs instead of fixed buffers

http: use strbufs instead of fixed buffers

We keep the names of incoming packs and objects in fixed
PATH_MAX-size buffers, and snprintf() into them. This is
unlikely to end up with truncated filenames, but it is
possible (especially on systems where PATH_MAX is shorter
than actual paths can be). Let's switch to using strbufs,
which makes the question go away entirely.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>