t: support clang/gcc AddressSanitizer

t: support clang/gcc AddressSanitizer

When git is compiled with "-fsanitize=address" (using clang
or gcc >= 4.8), all invocations of git will check for buffer
overflows. This is similar to running with valgrind, except
that it is more thorough (because of the compiler support,
function-local buffers can be checked, too) and runs much
faster (making it much less painful to run the whole test
suite with the checks turned on).

Unlike valgrind, the magic happens at compile-time, so we
don't need the same infrastructure in the test suite that we
did to support --valgrind. But there are two things we can
help with:

  1. On some platforms, the leak-detector is on by default,
     and causes every invocation of "git init" (and thus
     every test script) to fail. Since running git with
     the leak detector is pointless, let's shut it off
     automatically in the tests, unless the user has already
     configured it.

  2. When apache runs a CGI, it clears the environment of
     unknown variables. This means that the $ASAN_OPTIONS
     config doesn't make it to git-http-backend, and it
     dies due to the leak detector. Let's mark the variable
     as OK for apache to pass.

With these two changes, running

    make CC=clang CFLAGS=-fsanitize=address test

works out of the box.

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