submodule tests: test for "add <repository> <abs-path>"

Add a missing test for ""add <repository> <path>" where "<path>" is an
absolute path. This tests code added in [1] and later turned into an
"else" branch in clone_submodule() in [2] that's never been tested.

This needs to be skipped on WINDOWS because all of $PWD, $(pwd) and
the "$(pwd -P)" we get via "$submodurl" would fail in CI with e.g.:

	fatal: could not create directory 'D:/a/git/git/t/trash
	directory.t7400-submodule-basic/.git/modules/D:/a/git/git/t/trash
	directory.t7400-submodule-basic/add-abs'

I.e. we can't handle these sorts of paths in this context on that
platform.

I'm not sure where we run into the edges of "$PWD" behavior on
Windows (see [1] for a previous loose end on the topic), but for the
purposes of this test it's sufficient that we test this on other
platforms.

1. ee8838d157 (submodule: rewrite `module_clone` shell function in C,
   2015-09-08)
2. f8eaa0ba98 (submodule--helper, module_clone: always operate on
   absolute paths, 2016-03-31)

1. https://lore.kernel.org/git/220630.86edz6c75c.gmgdl@evledraar.gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7400-submodule-basic.sh

@@ -178,6 +178,11 @@ test_expect_success 'submodule add' '
 	test_must_be_empty untracked
 '
 
+test_expect_success !WINDOWS 'submodule add (absolute path)' '
+	test_when_finished "git reset --hard" &&
+	git submodule add "$submodurl" "$submodurl/add-abs"
+'
+
 test_expect_success 'setup parent and one repository' '
 	test_create_repo parent &&
 	test_commit -C parent one