t3310: extract common notes_merge_files_gone()

t3310: extract common notes_merge_files_gone()

We have many statements which are duplicated. Extract and replace these
duplicate statements with notes_merge_files_gone().

While we're at it, replace the test_might_fail(), which should only be
used on git commands.

Also, remove the redirection from stderr to /dev/null. This is because
the test scripts automatically suppress output already. Otherwise, if a
developer asks for verbose output via the `-v` flag, the stderr output
may be useful for debugging.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>