diff-delta: fold two special tests into one plus cleanups

diff-delta: fold two special tests into one plus cleanups

Testing for realloc and size limit can be done with only one test per
loop. Make it so and fix a theoretical off-by-one comparison error in
the process.

The output buffer memory allocation is also bounded by max_size when
specified.

Finally make some variable unsigned to allow the handling of files up to
4GB in size instead of 2GB.

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>