compat: let git_mmap use malloc(3) directly

compat: let git_mmap use malloc(3) directly

xmalloc() dies on error, allows zero-sized allocations and enforces
GIT_ALLOC_LIMIT for testing.  Our mmap replacement doesn't need any of
that.  Let's cut out the wrapper, reject zero-sized requests as required
by POSIX and use malloc(3) directly.  Allocation errors were needlessly
handled by git_mmap() before; this code becomes reachable now.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>