[vault] lock repository when working with it

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[vault] garbage collection action: gc

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[gc] vault-gc systemd service

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
[transfer] lock vault during the export

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
use migrated branch to mark migration was full

So, in the case of interrupted process it should be clear is
recreation of the history was completed

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
documenting gc code generator scripts

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>