Machine Git engine v2 migration preflight

RESULT=PASS_MACHINE_GIT_V2_MIGRATION_PREFLIGHT

Common engine v2:
0367907663f124316e7702ae7920ffe45f564e873e43bf47c96a118e9f7ccd74

Migration classes:
- VM100: V1_REQUIRES_UPGRADE
- VM101: COMMON_ENGINE_MISSING
- VM121: V2_CURRENT

Repository heads:
- VM100: 951958cb66386b6995863a22801d3bd3a80a2e9a
- VM101: f0aa09394d6d0622b103a128b955135941c1060c
- VM121: 93e0e5a31f0900a498b77d22e6a7f2ca5fd7fee0

Safe delivery contract:
- no tar archive
- no parent-directory entries
- stream one file to a temporary file
- verify SHA256
- set only target-file ownership and mode
- atomically replace the target with mv
- verify parent-directory metadata did not change

No target machine was modified during this preflight.