t3310: stop checking for reference existence via `test -f`

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