Fix race condition in doit.tools.create_folder

`create_folder` had a race condition: if many tasks try to create the same folder then it is possible that between checking directory existence and actual creation of the directory in the next line another task will create that directory.
Since [python 3.2](https://docs.python.org/3.4/library/os.html#os.makedirs) `os.makedirs` has nice `exist_ok` argument that will do the same thing but without such problems.

CHANGES

@@ -11,9 +11,11 @@ Changes
  - fix GH-#99: positional argument on tasks not specified from cmd-line
  - fix GH-#97: `list` command does not display task-doc for `DelayedTask`
                 when `creates` is specified
+ - fix GH-#131: race condition in doit.tools.create_folder
  - GH-#114: `file_dep`, `targets` and `CmdAction` support pathlib.
 
 
+
 0.29.0 (*2015-08-16*)
 =====================
 

doit/tools.py

@@ -15,8 +15,7 @@
 # action
 def create_folder(dir_path):
     """create a folder in the given path if it doesnt exist yet."""
-    if not (os.path.exists(dir_path) and os.path.isdir(dir_path)):
-        os.makedirs(dir_path)
+    os.makedirs(dir_path, exist_ok=True)
 
 
 # title