[RFC] fast-import: note deletion command

Hi,

fast-import is able to write notes provided a command N <note_content_data_specification> <commit_specification>

There is no documented command to delete a note. It can't be easily replaced with Delete command on notes tree because of notes fanout.

But fast-import interprets 0{40} data_specification as a note deletion command. It is consistent with fast-import internals where null_sha1 is used for absent/unknown object. But is not consistent with file Modify command nor looks like a common convention in git ui.

So the questions are:
- should 0{40} not be treated as deletion toggle? Downside is that
it is used as one in t/t9301-fast-import-notes.sh.
- how should a documented way to delete notes look like?
ND commit_sha1 # One may think there are also NC, NR, NM
N delete commit_sha1
# Only :mark, full-40-byte-sha1 and 'inline' are allowed currently.
# So no clashes arise, but then one may also want M 'delete' path
# command to work too.