shortlog: use strbufs to read from stdin

shortlog: use strbufs to read from stdin

We currently use fixed-size buffers with fgets(), which
could lead to incorrect results in the unlikely event that a
line had something like "Author:" at exactly its 1024th
character.

But it's easy to convert this to a strbuf, and because we
can reuse the same buffer through the loop, we don't even
pay the extra allocation cost.

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