Use a file to ignore subvolumes when bootstrapping

When a file with a special name, .btrfish-ignore, exists in the root
of a subvolume, this subvolume is skipped during the bootstrapping
process.

bootstrap.fish

@@ -22,8 +22,12 @@ for pair in (get-subvol-mounts)
 	set safepath $subvoldir/$safename
 
 	if not test -d $safepath
-		sudo btrfs subvolume snapshot -r $mount $safepath
-		set snapshots $snapshots $safepath
+		if not test -f $mount/.btrfish-ignore
+			sudo btrfs subvolume snapshot -r $mount $safepath
+			set snapshots $snapshots $safepath
+		else
+			echo ".btrfish-ignore found in $mount, skipping..."
+		end
 	end
 end