Use Btrfs snapshots for chroot copies, when available

This is much faster than using Rsync to clone.

Rsync stays available when the chroots are not on a Btrfs.

archbuild

@@ -45,6 +45,7 @@ if ${clean_first} || [ ! -d "${chroots}/${repo}-${arch}" ]; then
 		exec 9>${copy}.lock
 		flock 9
 
+		{ type -P btrfs && btrfs subvolume delete ${copy}; } &>/dev/null
 		rm -rf ${copy}
 	done
 	exec 9>&-

makechrootpkg

@@ -127,8 +127,18 @@ if [ ! -d "$copydir" -o "$clean_first" -eq "1" ]; then
 	fi
 
 	echo -n 'creating clean working copy...'
-	mkdir -p "$copydir"
-	rsync -a --delete -q -W -x "$chrootdir/root/" "$copydir"
+	use_rsync=false
+	if type -P btrfs >/dev/null; then
+		[ -d $copydir ] && btrfs subvolume delete "$copydir" &>/dev/null
+		btrfs subvolume snapshot "$chrootdir/root" "$copydir" &>/dev/null || use_rsync=true
+	else
+		use_rsync=true
+	fi
+
+	if $use_rsync; then
+		mkdir -p "$copydir"
+		rsync -a --delete -q -W -x "$chrootdir/root/" "$copydir"
+	fi
 	echo 'done'
 
 	# Drop the read lock again

mkarchroot

@@ -181,6 +181,10 @@ else
 		exit 1
 	fi
 
+	if { type -P btrfs && btrfs subvolume create "${working_dir}"; } &>/dev/null; then
+		chmod 0755 "${working_dir}"
+	fi
+
 	mkdir -p "${working_dir}/var/lib/pacman/sync"
 	mkdir -p "${working_dir}/etc/"