Use common "revision.h" header for both fsck and rev-tree.

Use common "revision.h" header for both fsck and rev-tree.

It's really a very generic thing: the notion of one sha1 revision
referring to another one. "fsck" uses it for all nodes, and "rev-tree"
only tracks commit-node relationships, but the code was already
the same - now we just make that explicit by moving it to a common
header file.