gc doesn't know about by-sha256

[allisonkarlitskaya]

Running gc in the presence of by-sha256 splitstreams gives:

Error: category directory contains non-symlink

Which is probably for the best, because it doesn't know about them and would probably decide to delete them if it could find them.

We should do something sensible there. I wonder if we should take a more generic approach to symlink handling: all symlinks reachable from anything under */refs/** get to live. We can better do the checking about if the structure is correct from fsck.

[allisonkarlitskaya]

by-sha256 got axed, but we still need to update the GC code to understand that the symlinks directly in streams/ are now body sha256.