ci: run tests with SHA-256

ci: run tests with SHA-256

Now that we have Git supporting SHA-256, we'd like to make sure that we
don't regress that state.  Unfortunately, it's easy to do so, so to
help, let's add code to run one of our CI jobs with SHA-256 as the
default hash.  This will help us detect any problems that may occur.

We pick the linux-clang job because it's relatively fast and the
linux-gcc job already runs the testsuite twice.  We want our tests to
run as fast as possible, so we wouldn't want to add a third run to the
linux-gcc job.  To make sure we properly exercise the code, let's run
the tests in the default mode (SHA-1) first and then run a second time
with SHA-256.  We explicitly specify SHA-1 for the first run so that if
we change the default in the future, we make sure to test both cases.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>