Makefile: suppress a sparse warning for pack-revindex.c

Makefile: suppress a sparse warning for pack-revindex.c

Sparse has, for a long time, been issuing the following warning against
the pack-revindex.c file:

      SP pack-revindex.c
  pack-revindex.c:64:23: warning: memset with byte count of 262144

This results from a unconditional check, with a hard-coded limit, which
is really only appropriate for the kernel source code. (The check is for
a 'large' byte count in a call to memcpy(), memset(), copy_from_user()
and copy_to_user() functions).

A recent release of sparse (v0.5.1) has introduced some options to allow
this check to be turned off (-Wno-memcpy-max-count) or to specify the
actual limit used (-fmemcpy-max-count=COUNT), rather than a hard-coded
limit of 100000.

In order to suppress the warning, add a target for pack-revindex.sp that
adds the '-Wno-memcpy-max-count' option to the SPARSE_FLAGS variable.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>