Merge branch 'jk/build-with-asan'

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

diff --cc Makefile
Simple merge

diff --cc t/test-lib.sh
Simple merge