t: default to compile-time default hash if not set

Right now, the default compile-time hash is SHA-1. However, in the future, this might change and it would be helpful to gracefully handle this case in our testsuite. To avoid making these assumptions, let's introduce a variable that contains the built-in default hash and use it in our setup code as the fallback value if no hash was explicitly set. For now, this is always SHA-1, but in a future commit, we'll allow adjusting this and the variable will be more useful. To allow us to make our tests more robust, allow test_oid to take the --hash=builtin option to specify this hash, whatever it is. Additionally, add a DEFAULT_HASH_ALGORITHM prerequisite to check for the compile-time hash. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>