fast-import: Fix incorrect fanout level when modifying existing notes refs

fast-import: Fix incorrect fanout level when modifying existing notes refs

This fixes the bug uncovered by the tests added in the previous two patches.

When an existing notes ref was loaded into the fast-import machinery, the
num_notes counter associated with that ref remained == 0, even though the
true number of notes in the loaded ref was higher. This caused a fanout
level of 0 to be used, although the actual fanout of the tree could be > 0.
Manipulating the notes tree at an incorrect fanout level causes removals to
silently fail, and modifications of existing notes to instead produce an
additional note (leaving the old object in place at a different fanout level).

This patch fixes the bug by explicitly counting the number of notes in the
notes tree whenever it looks like the num_notes counter could be wrong (when
num_notes == 0). There may be false positives (i.e. triggering the counting
when the notes tree is truly empty), but in those cases, the counting should
not take long.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>