subtree: allow testing with reftable backend

"git subtree" (in contrib/) comes with its own test script, which
has this line

    defaultBranch=$(sed "s,ref: refs/heads/,," "$test_count/.git/HEAD")

that assumes that you can read from .git/HEAD as a regular text file
and you'd find a textual symref in reffiles backend.

Not necessarily.

    make && cd contrib/subtree &&
    GIT_TEST_DEFAULT_REF_FORMAT=reftable make test

fails due to this.  Use "git symbolic-ref" instead to read the value
of the symref.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

contrib/subtree/t/t7900-subtree.sh

@@ -1597,7 +1597,8 @@ test_expect_success 'push split to subproj' '
 
 test_expect_success 'subtree descendant check' '
 	subtree_test_create_repo "$test_count" &&
-	defaultBranch=$(sed "s,ref: refs/heads/,," "$test_count/.git/HEAD") &&
+
+	defaultBranch=$(git -C "$test_count" symbolic-ref --short HEAD) &&
 	test_create_commit "$test_count" folder_subtree/a &&
 	(
 		cd "$test_count" &&