Enable SHA-256 by default in breaking changes mode

Our document on breaking changes indicates that we intend to default to
SHA-256 in Git 3.0.  Since most people choose the default option, this
is an important security upgrade to our defaults.

To allow people to test this case, when WITH_BREAKING_CHANGES is set in
the configuration, build Git with SHA-256 as the default hash.  Update
the testsuite to use the build options information to automatically
choose the right value.

Note that if the command substitution for GIT_TEST_BUILTIN_HASH fails,
so does the testsuite—and quite spectacularly at that.  Thus, the case
where the Git binary is somehow subtly broken will not go undetected.

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

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