test-delta: read input into a heap buffer

We currently read the input to test-delta by mmap()-ing it. However, memory-checking tools like valgrind and ASan are less able to detect reads/writes past the end of an mmap'd buffer, because the OS is likely to give us extra bytes to pad out the final page size. So instead, let's read into a heap buffer. As a bonus, this also makes it possible to write tests with empty bases, as mmap() will complain about a zero-length map. This is based on a patch by Jann Horn <jannh@google.com> which actually aligned the data at the end of a page, and followed it with another page marked with mprotect(). That would detect problems even without a tool like ASan, but it was significantly more complex and may have introduced portability problems. By comparison, this approach pushes the complexity onto existing memory-checking tools. Note that this could be done even more simply by using strbuf_read_file(), but that would defeat the purpose: strbufs generally overallocate (and at the very least include a trailing NUL which we do not care about), which would defeat most memory checkers. Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: Nicolas Pitre <nico@fluxnic.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>