Makefile: turn on NO_MMAP when building with LSan

Makefile: turn on NO_MMAP when building with LSan

The past few commits fixed some cases where we leak memory allocated by
mmap(). Building with SANITIZE=leak doesn't detect these because it
covers only heap buffers allocated by malloc().

But if we build with NO_MMAP, our compat mmap() implementation will
allocate a heap buffer and pread() into it. And thus Lsan will detect
these leaks for free.

Using NO_MMAP is less performant, of course, since we have to use extra
memory and read in the whole file, rather than faulting in pages from
disk. But LSan builds are already slow, and this doesn't make them
measurably worse. Getting extra coverage for our leak-checking is worth
it.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>