debug tracing of steps

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
basic rollback on the first stage of migration

tag relocation is unlikely to fail

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
allow empty commits to avoid errors in corner cases

It is safer to get empty commit than to process corner cases, proved
also by testing :)

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