max steps for merge

[gsmolk]

It would greate to have an option to set the requirement for minimun length of backup chain for retention merge.
--merge-min-depth=num

[gsmolk]

DELTA4 <- Active
DELTA4 <- Active
DELTA3 <- Active
--- retention window
DELTA2 <- Expired
DELTA1 <- Expired
FULL <- Expired

Example:
If running retention merge with --merge-min-depth=4, then nothing should be merged, until DELTA3 expires also