shallow.c: make paint_alloc slightly more robust

shallow.c: make paint_alloc slightly more robust

paint_alloc() allocates a big block of memory and splits it into
smaller, fixed size, chunks of memory whenever it's called. Each chunk
contains enough bits to present all "new refs" [1] in a fetch from a
shallow repository.

We do not check if the new "big block" is smaller than the requested
memory chunk though. If it happens, we'll happily pass back a memory
region smaller than expected. Which will lead to problems eventually.

A normal fetch may add/update a dozen new refs. Let's stay on the
"reasonably extreme" side and say we need 16k refs (or bits from
paint_alloc's perspective). Each chunk of memory would be 2k, much
smaller than the memory pool (512k).

So, normally, the under-allocation situation should never happen. A bad
guy, however, could make a fetch that adds more than 4m new/updated refs
to this code which results in a memory chunk larger than pool size.
Check this case and abort.

Noticed-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Reviewed-by: Jeff King <peff@peff.net>
[1] Details are in commit message of 58babff (shallow.c: the 8 steps to
    select new commits for .git/shallow - 2013-12-05), step 6.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>