merge(third_party/git): Merge squashed git subtree at v2.23.0

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third_party/git/t/interop/README

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+Git version interoperability tests
+==================================
+
+This directory has interoperability tests for git. Each script is
+similar to the normal test scripts found in t/, but with the added twist
+that two special versions of git, "git.a" and "git.b", are available in
+the PATH. Individual tests can then check the interaction between the
+two versions.
+
+When you add a feature that handles backwards compatibility between git
+versions, it's encouraged to add a test here to make sure it behaves as
+you expect.
+
+
+Running Tests
+-------------
+
+The easiest way to run tests is to say "make".  This runs all
+the tests against their default versions.
+
+You can run a single test like:
+
+    $ ./i0000-basic.sh
+    ok 1 - bare git is forbidden
+    ok 2 - git.a version (v1.6.6.3)
+    ok 3 - git.b version (v2.11.1)
+    # passed all 3 test(s)
+    1..3
+
+Each test contains default versions to run against. You may override
+these by setting `GIT_TEST_VERSION_A` and `GIT_TEST_VERSION_B` in the
+environment. Note that not all combinations will give sensible outcomes
+for all tests (e.g., a test checking for a specific old/new interaction
+may want something "old" enough" and something "new" enough; see
+individual tests for details).
+
+Version names should be resolvable as revisions in the current
+repository. They will be exported and built as needed using the
+config.mak files found at the root of your working tree.
+
+The exception is the special version "." which uses the currently-built
+contents of your working tree.
+
+You can set the following variables (in the environment or in your config.mak):
+
+    GIT_INTEROP_MAKE_OPTS
+	Options to pass to `make` when building a git version (e.g.,
+	`-j8`).
+
+You can also pass any command-line options taken by ordinary git tests (e.g.,
+"-v").
+
+
+Naming Tests
+------------
+
+The interop test files are named like:
+
+	iNNNN-short-description.sh
+
+where N is a decimal digit.  The same conventions for choosing NNNN as
+for normal tests apply.
+
+
+Writing Tests
+-------------
+
+An interop test script starts like a normal script, declaring a few
+variables and then including interop-lib.sh (which includes test-lib.sh).
+Besides test_description, you should also set the $VERSION_A and $VERSION_B
+variables to give the default versions to test against. See t0000-basic.sh for
+an example.
+
+You can then use test_expect_success as usual, with a few differences:
+
+  1. The special commands "git.a" and "git.b" correspond to the
+     two versions.
+
+  2. You cannot call a bare "git". This is to prevent accidents where
+     you meant "git.a" or "git.b".
+
+  3. The trash directory is _not_ a git repository by default. You
+     should create one with the appropriate version of git.
+
+At the end of the script, call test_done as usual.