We generate range strings like "1234abcd...5678efab" for use
in the the fetch and push status tables. We use fixed-size
buffers along with strcat to do so. These aren't buggy, as
our manual size computation is correct, but there's nothing
checking that this is so.  Let's switch them to strbufs
instead, which are obviously correct, and make it easier to
audit the code base for problematic calls to strcat().
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>