Use GNU Parallel if available to speed up updating

dup

@@ -146,10 +146,16 @@ then
 
     targets="$(find * -maxdepth 0 -type d -execdir test -e "{}/.git" \; -print)"
 
-    for dir in $targets
-    do
-	    do_pull_maybe_quiet "$dir"
-    done
+    if type parallel &> /dev/null
+    then
+        export -f do_pull do_pull_maybe_quiet
+        parallel --max-args 1 do_pull_maybe_quiet ::: $targets
+    else
+        for dir in $targets
+        do
+	        do_pull_maybe_quiet "$dir"
+        done
+    fi
 
     maybe_stow $targets
 fi