…ot just in protected refs' trees. Fixed rtyley#49, rtyley#53, rtyley#138.

Added objectId exclusion filters during tree blob-cleaning, such that blobs that exist in the trees of protected refs as stored in the census (AKA dirt) are protected not only in those trees, but in any other trees in which they occur in in the walked-history.  This prevents the perception that those files are being deleted in-history and then re-added in the final re-written commit, which is the protected HEAD (with dirt) and its untouched tree.  This is more a perception because Git does not track the history of individual files, but it does show diffs and logs that indicate such changes, and the behaviour prior to this change is to remove protected blobs from non-protected history trees, retaining them only in the final HEAD ref, which then shows as an add in that commit when logs/diffs are taken.

This change is convenient if your clean-up selectors (by name, or size) do select some dirt (files still in HEADs that you want to keep) but you don't want those files to appear as if they were recently added to HEAD.