t1401: stop treating FETCH_HEAD as real reference

One of the tests in t1401 asserts that we can create a symref from a
symbolic reference to a top-level reference, which is done by linking
from `refs/heads/top-level` to `FETCH_HEAD`. But `FETCH_HEAD` is not a
proper reference and doesn't even follow the loose reference format, so
it is not a good candidate for the logic under test.

Refactor the test to use `ORIG_HEAD` instead of `FETCH_HEAD`. This also
works with other backends than the reffiles one.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

t/t1401-symbolic-ref.sh

@@ -171,8 +171,8 @@ test_expect_success 'symbolic-ref refuses invalid target for non-HEAD' '
 '
 
 test_expect_success 'symbolic-ref allows top-level target for non-HEAD' '
-	git symbolic-ref refs/heads/top-level FETCH_HEAD &&
-	git update-ref FETCH_HEAD HEAD &&
+	git symbolic-ref refs/heads/top-level ORIG_HEAD &&
+	git update-ref ORIG_HEAD HEAD &&
 	test_cmp_rev top-level HEAD
 '