t3305: check notes fanout more carefully and robustly

t3305: check notes fanout more carefully and robustly

In short, before this patch, this test script:
 - creates many notes
 - verifies that all notes in the notes tree has a fanout of 1
 - removes most notes
 - verifies that the notes in the notes tree now has a fanout of 0

The fanout verification only happened twice: after creating all the
notes, and after removing most of them.

This patch strengthens the test by checking the fanout after _each_
added/removed note: We assert that the switch from fanout 0 -> 1
happens exactly once while adding notes (and that the switch pervades
the entire notes tree). Likewise, we assert that the switch from
fanout 1 -> 0 happens exactly once while removing notes.

Additionally, we decrease the number of notes left after removal,
from 50 to 15 notes, in order to ensure that fanout 1 -> 0 transition
keeps happening regardless of external factors[1].

[1]: Currently (with the SHA1 hash function and the deterministic
object ids of the test environment) the fanout heuristic in the notes
code happens to switch from 0 -> 1 at 109 notes, and from 1 -> 0 at
59 notes. However, changing the hash function or other external
factors will vary these numbers, and the latter may - in theory - go
as low as 15. For more details, please see the discussion at
https://public-inbox.org/git/20200125230035.136348-4-sandals@crustytoothpaste.net/

Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Brian M. Carlson <sandals@crustytoothpaste.net>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>