t: allow specifying compatibility hash

We want to specify a compatibility hash for testing interactions for
SHA-256 repositories where we have SHA-1 compatibility enabled.  Allow
the user to specify this scenario in the test suite by setting
GIT_TEST_DEFAULT_HASH to "sha256:sha1".

Note that this will get passed into GIT_DEFAULT_HASH, which Git itself
does not presently support.  However, we will support this in a future
commit.

Since we'll now want to know the value for a specific version, let's add
the ability to specify either the storage hash (in this case, SHA-256)
or the compatibility hash (SHA-1).  We use a different value for the
compatibility hash that will be enabled for all repositories
(test_repo_compat_hash_algo) versus the one that is used individually in
some tests (test_compat_hash_algo), since we want to still run those
individual tests without requiring that the testsuite be run fully in a
compatibility mode.

In some cases, we'll need to adjust our test suite to work in a proper
way with a compatibility hash.  For example, in such a case, we'll only
use pack index v3, since v1 and v2 lack support for multiple algorithms.
Since we won't want to write those older formats, we'll need to skip
tests that do so.  Let's add a COMPAT_HASH prerequisite for this
purpose.

Finally, in this scenario, we can no longer rely on having broken
objects work since we lack compatibility mappings to rewrite objects in
the repository.  Add a prerequisite, BROKEN_OBJECTS, that we define in
terms of COMPAT_HASH and checks to see if creating deliberately broken
objects is possible, so that we can disable these tests if not.

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

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