Add a lua script to clean the aliases file

Note that cmder doesn't ship with lua. Next step is converting this
script to perl, which ships with msysgit.

bin/alias.bat

@@ -15,6 +15,7 @@ if not ["%_temp%"] == ["%_temp2%"] (
 
 echo %* >> "%CMDER_ROOT%\config\aliases"
 doskey /macrofile="%CMDER_ROOT%\config\aliases"
+lua "%CMDER_ROOT%\scripts\clean_aliases.lua"
 echo Alias created
 endlocal
 goto:eof

scripts/clean_aliases.lua

@@ -0,0 +1,63 @@
+--[[
+Cmder adds aliases to its aliases file without caring for duplicates.
+This can result in the aliases file becoming bloated. This script cleans
+the aliases file.
+]]
+
+local aliases = {}
+local alias_file = os.getenv('CMDER_ROOT') .. "/config/aliases"
+
+-- from http://www.lua.org/pil/19.3.html
+-- function to create an iterator that returns the key-value pairs
+-- sorted by keys
+local function pairsByKeys (t, f)
+
+    local a = {}
+    for n in pairs(t) do
+        table.insert(a, n)
+    end
+    table.sort(a, f)
+
+    local i = 0                 -- iterator variable
+    local iter = function ()    -- iterator function
+        i = i + 1
+        if a[i] == nil then
+            return nil
+        else
+            return a[i], t[a[i]]
+        end
+    end
+
+    return iter
+end
+
+-- First step
+-- Read the aliases file line by line, and put every entry in
+-- a dictionary. The newer aliases being the last, the new will
+-- always be kept over the old.
+for line in io.lines(alias_file) do
+
+    -- Doskey actually accepts a lot of weird characters in macros
+    -- definitions.
+    local key, value = line:match('([^=%s<>]+)=(.*)')
+
+    if key then
+        aliases[key] = value
+    else
+        print('Invalid macro definition:   '..line)
+    end
+
+end
+
+-- Second step
+-- Write back the aliases. Sort them to make the file look nice.
+local f = io.open(alias_file, 'w')
+
+for key, value in pairsByKeys(aliases) do
+    -- write the pair only if the value is not empty
+    if value then
+        f:write(key .. '=' .. value .. '\n')
+    end
+end
+
+f:close()