vfs: include unique repo id in resolution cache key

vfs: include unique repo id in resolution cache key

...since resolve() currently requires a full parent path and the root
refs are only applicable to a particular repository.

Use differing integers to identify repositories that may be
independent (with respect to refs, tags, etc.), and use (typically
small) integers rather than the repo path/address so that they'll be
short if we want to embed them directly in cache keys later.

Use realpath() for local repositories in order to detect when the same
repository is reachable by multiple paths.  (Something similar could
eventually be done for remotes.)

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>