Compress decimal escapes for string.format("%q").

Lua 5.2 escapes all control characters using decimal escape
sequences. The previous code did the same but always used three
decimals (possibly zero-padded) no matter the following character.
The new version only uses all three decimals when another decimal
character is following.

compat52.lua

@@ -540,8 +540,8 @@ if _VERSION == "Lua 5.1" then
             ["\""] = "\\\""
          }
 
-         local function addquoted(c)
-            return addqt[c] or string_format("\\%03d", c:byte())
+         local function addquoted(c, d)
+            return (addqt[c] or string_format(d~= "" and "\\%03d" or "\\%d", c:byte()))..d
          end
 
          function string.format(fmt, ...)
@@ -553,7 +553,7 @@ if _VERSION == "Lua 5.1" then
                   if kind == "s" then
                      args[i] = _tostring(args[i])
                   elseif kind == "q" then
-                     args[i] = '"'..string_gsub(args[i], "[%z%c\\\"\n]", addquoted)..'"'
+                     args[i] = '"'..string_gsub(args[i], "([%z%c\\\"\n])(%d?)", addquoted)..'"'
                      return lead.."%"..mods.."s"
                   end
                end

tests/test.lua

@@ -326,7 +326,7 @@ do
          return _tostring(v)
       end
    end
-   print("string.format()", string.format("%q", "\"\\\0000\0010\r0\n0\t0\""))
+   print("string.format()", string.format("%q", "\"\\\0000\0010\002\r\n0\t0\""))
    print("string.format()", string.format("%12.3fx%%sxx%.6s", 3.1, {}))
    print("string.format()", string.format("%-3f %%%s %%s", 3.1, true))
    print("string.format()", string.format("% 3.2g %%d %%%s", 3.1, nil))