fix >4GiB source delta assertion failure

fix >4GiB source delta assertion failure

When people try insane things such as delta-compressing 4GiB files, they
get this assertion:

diff-delta.c:285: create_delta_index: Assertion `packed_entry - (struct index_entry *)mem == entries' failed.

This happens because:

1) the 'entries' variable is an unsigned int

2) it is assigned with entries = (bufsize - 1) / RABIN_WINDOW
   (that itself is not a problem unless bufsize > 4G * RABIN_WINDOW)

3) the buffer is indexed from top to bottom starting at
   "data = buffer + entries * RABIN_WINDOW" and the multiplication
   here does indeed overflows, making the resulting top of the buffer
   much lower than expected.

This makes the number of actually produced index entries smaller than
what was computed initially, hence the assertion.

Furthermore, the current delta encoding format cannot represent offsets
into a reference buffer with more than 32 bits anyway.  So let's just
limit the number of entries to what the delta format can encode.

Reported-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>