Match directories only for known in the current tree keys

[yarikoptic]

Follow up for a possible "robust" operation while figuring out either we should state that directory under .git/annex/objects "exists" or not, in follow up to https://github.com/datalad/datalad-fuse/pull/52#issuecomment-1027049883

Ideally we should get a full filetree in current HEAD, get all git-annex'ed files (might be locked?) and only react to those directories under .git/annex/objects which are actually used.

Pros:

• more "specific"

Cons:

• heavy tree could take awhile to read
• heavier checks for every such directory/file