apply: reject patches larger than ~1 GiB

apply: reject patches larger than ~1 GiB

The apply code is not prepared to handle extremely large files. It uses
"int" in some places, and "unsigned long" in others.

This combination leads to unfortunate problems when switching between
the two types. Using "int" prevents us from handling large files, since
large offsets will wrap around and spill into small negative values,
which can result in wrong behavior (like accessing the patch buffer with
a negative offset).

Converting from "unsigned long" to "int" also has truncation problems
even on LLP64 platforms where "long" is the same size as "int", since
the former is unsigned but the latter is not.

To avoid potential overflow and truncation issues in `git apply`, apply
similar treatment as in dcd1742e56 (xdiff: reject files larger than
~1GB, 2015-09-24), where the xdiff code was taught to reject large
files for similar reasons.

The maximum size was chosen somewhat arbitrarily, but picking a value
just shy of a gigabyte allows us to double it without overflowing 2^31-1
(after which point our value would wrap around to a negative number).
To give ourselves a bit of extra margin, the maximum patch size is a MiB
smaller than a full GiB, which gives us some slop in case we allocate
"(records + 1) * sizeof(int)" or similar.

Luckily, the security implications of these conversion issues are
relatively uninteresting, because a victim needs to be convinced to
apply a malicious patch.

Reported-by: 정재우 <thebound7@gmail.com>
Suggested-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>