Add more testcases to test fast-import of notes

This patch adds testcases verifying correct behaviour in several scenarios
regarding fast-import of notes:
- using a mixture of 'N' and 'M' commands
- updating existing notes
- concatenation of notes
- 'deleteall' also removes notes
- fanout schemes is added/removed when needed
- git-fast-import's branch unload/reload preserves notes
- non-notes are not clobbered in the presence of notes

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