Makefile

The build procedure has been improved to allow building and testing
Git with address sanitizer more easily.

* jk/build-with-asan:
  Makefile: disable unaligned loads with UBSan
  Makefile: turn off -fomit-frame-pointer with sanitizers
  Makefile: add helper for compiling with -fsanitize
  test-lib: turn on ASan abort_on_error by default
  test-lib: set ASAN_OPTIONS variable before we run git