pretty: drop strbuf pre-sizing from add_rfc2047()

pretty: drop strbuf pre-sizing from add_rfc2047()

At the top of add_rfc2047() we do this:

  strbuf_grow(sb, len * 3 + strlen(encoding) + 100);

where "len" is the size of the header (like an author name) we are about
to encode into the buffer. This pre-sizing is purely an optimization; we
use strbuf_addf() and friends to actually write into the buffer, and
they will grow the buffer as necessary.

But there's a problem with the code above: the input can be arbitrarily
large, so we might overflow a size_t while doing that computation,
ending up with a too-small allocation request. Overflowing requires an
impractically large input on a 64-bit system, but is easy to demonstrate
on a 32-bit system with a commit whose author name is ~1.4GB.

Because this pre-sizing is just an optimization, there's no real harm.
We'll start with a smaller buffer and grow it as necessary. But it
_looks_ like a vulnerability, since some other code may pre-size a
strbuf and then write directly into its buffer. So it's worth avoiding
the overflow in the first place.

The obvious way to do that is via checked operations like st_add() and
friends. But taking a step back, is this pre-sizing actually helping
anything?

The computation goes all the way back to 4234a76167 (Extend
--pretty=oneline to cover the first paragraph,, 2007-06-11), but back
then we really were sizing the array to write into directly! In
674d172730 (Rework pretty_print_commit to use strbufs instead of custom
buffers., 2007-09-10) that switched to a strbuf, and at that point it
was a pure optimization.

Is the optimization helping? I don't think so. Even for a gigantic case
like the 1.4GB author name, I couldn't measure any slowdown when
removing it. And most input will be much smaller, and added to a running
strbuf containing the rest of the email-header output. We can just rely
on strbuf's usual amortized-linear growth.

So deleting the line seems like the best way to go. It eliminates the
integer overflow and makes the code a tiny bit simpler.

Reported-by: Luke Martin <lmartin@paramenoeng.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>