Add "bup cat-file [--meta|--bupm] /branch/revision/[path]".

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
metadata_file(): don't load metadata objects for all the files in a dir.

When only asked for the .bupm via metdata_file(), don't also parse the
.bupm and create all the metadata objects for the dir.  Aside from
being potentially wasteful, that may also make it impossible to
retrieve a broken .bupm via "cat-file --bupm" (for analysis).

Thanks to Anton Eliasson <devel@antoneliasson.se> for helping find the
problem and test the fix.

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