Extend object reference syntax with size assertions.

Object references can now include a size assertion, such as [=1024]
which indicates that the referenced object is exactly 1024 bytes in
length.  If a metadata log or statcache file is produced using this
reference form where appropriate, then it should be possible to rebuild
much of the object index in the local database (by looking for files
which are unchanged and computing hashes of blocks from that file where
it is known that an entire object was used, not just a fragment of an
object).

This commit merely adds support for parsing the new references; they are
not yet generated by any code.