t3310: stop checking for reference existence via test -f

One of the tests in t3310 exercises whether the special references
`NOTES_MERGE_PARTIAL` and `NOTES_MERGE_REF` exist as expected when the
notes subsystem runs into a merge conflict. This is done by checking
on-disk data structures directly though instead of asking the reference
backend.

Refactor the test to use git-rev-parse(1) instead.

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

Author: pks-t

t/t3310-notes-merge-manual-resolve.sh

@@ -561,9 +561,9 @@ y and z notes on 4th commit
 EOF
 	# Fail to finalize merge
 	test_must_fail git notes merge --commit >output 2>&1 &&
-	# .git/NOTES_MERGE_* must remain
-	test -f .git/NOTES_MERGE_PARTIAL &&
-	test -f .git/NOTES_MERGE_REF &&
+	# NOTES_MERGE_* refs and .git/NOTES_MERGE_* state files must remain
+	git rev-parse --verify NOTES_MERGE_PARTIAL &&
+	git rev-parse --verify NOTES_MERGE_REF &&
 	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha1 &&
 	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha2 &&
 	test -f .git/NOTES_MERGE_WORKTREE/$commit_sha3 &&