Running ipfs pin on similar dags should be almost instant but its slow

#### Version information:

# ipfs version --all
go-ipfs version: 0.4.10-
Repo version: 5
System version: amd64/linux
Golang version: go1.8.3

#### Type: Enhancment
#### Severity:
Medium - A non-essential functionality does not work, performance issues, etc.

#### Description:

I've been repeatedly pinning to my vps a "public" directory in my home dir for some time now. Each time I do so, it goes through "Fetched/Processed 80212 nodes" even though 99% of those nodes are already pinned on my vps. Rather than processing 80212 nodes, IPFS should realize when a branch of the merkledag has already been pinned and stop.