dupe and comment: format and comment new "do" implementation

i find it easier to read when the different parts are separated,
and while i have done one-line `if this then do that end`s before,
i've only ever done it for short operations where it's immediately
obvious what's going on and you don't need to do any thinking whatsoever.

e.g. this function from depctrl config:
        local function get_log_level(num)
                if num == 0 then return "0: Fatal"
                elseif num == 1 then return "1: Error"
                elseif num == 2 then return "2: Warning"
                elseif num == 3 then return "3: Hint"
                elseif num == 4 then return "4: Debug"
                elseif num == 5 then return "5: Trace" end
                return nil
        end
I don't think the bits that fix sel and act count, at least not without comments.

speaking of comments, i've added some.
i hope i've understood what's going on correctly, please feel free to correct me if not.

macros/garret.dupe-and-comment.lua

@@ -13,11 +13,20 @@ end
 
 local function comment(subs, sel, act)
     for i=#sel,1,-1 do
-        local line=subs[sel[i]]
-        line.comment = true
-        subs.insert(sel[i]+1, line)
-        for j=i+1,#sel do sel[j] = sel[j] + 1 end
-        if act > sel[i] then act = act + 1 end
+        local line=subs[sel[i]] -- grab copy of current line
+
+        -- now use that copy for a different line
+        line.comment = true -- comment out the new dupe line
+        subs.insert(sel[i]+1, line) -- and put it below
+
+        -- sort out selection
+        for j=i+1,#sel do
+            sel[j] = sel[j] + 1 -- bump all of sel by 1 to compensate for new line
+        end -- first item isnt included because it's not affected
+
+        if act > sel[i] then -- if we've not got to the active line yet
+            act = act + 1 -- bump by 1 to compensate for new lines above
+        end
     end
     aegisub.set_undo_point(script_name)
     return sel, act