Fixed move handling when caught in a relocate

This was a surprisingly tricky issue. One of the subtle requirements for
the new move handling to work is that the block containing the move does
not change until the move is resolved. Initially, this seemed easy to
implement, given that a move is always immediately followed by its
resolution.

However, the extra metadata-pair operations needed to maintain integrity
present a challenge. At any commit, a directory block may end up moved
as a side effect of relocation due to a bad block.

The fix here is to move the move resolution directly into the commit
logic. This means that any commit to a block containing a move will be
implicitly resolved, leaving the later attempt at move resolution as a
noop.

This fix required quite a bit of restructuring, but as a nice
side-effect some of the complexity around moves actually went away.
Additionally, the new move handling is surprisingly powerful at
combining moves with nearby commits. And we now get same-metadata-pair
renames for free! A win for procrasination on that minor feature.

Author: geky