vfs: read metadata only if needed

vfs: read metadata only if needed

If we get into contents(), we have an indication of whether or
not metadata is needed (the want_meta argument), but it doesn't
get passed down to revlist_items() and further down, so that in
cache_commit() we eventually call _revlist_item_from_oid() with
metadata always. This is wasteful.

Fix this by passing the information all the way down.

To make the caching work properly in this case, store a special
entry in the revlist dict (_HAS_META_ENTRY) indicating whether
or not metadata was cached, and if that's not set on a later
lookup, then don't return it if it doesn't have metadata but we
need the metadata.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>