Add current-filesystem and path-filesystems to t/lib.sh; use to guard test.

Add current-filesystem and path-filesystems to t/lib.sh; use to guard test.

Add current-filesystem which reports the current filesystem type, and
path-filesystems which reports all the filesystems back up to the
root, and use the latter to skip the "save --strip-path (no match)"
test whenever all the filesystems involved aren't the same type.

This issue was discovered while testing on
/some/ext4/tree/containing/a/btrfs/tree.  When the test indexes/saves,
it picks up the metadata for the parents above the btrfs, which may
have (for example) linux +e attrs.  Then when it tries to restore the
tree fully within the btrfs, the restore defers an error when it tries
to restore the +e, which isn't valid for btrfs.

Normally we can avoid this problem via --strip, but this particular
test is actually checking the behavior when --strip doesn't apply.

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