Fix mkdir race condition in LooseObjectDB.store

This replaces the conditional call to os.mkdir that raises an
unintended FileExistsError if the directory is created between the
check and the os.mkdir call, using a single os.makedirs call
instead, with exist_ok=True.

This way, we attempt creation in a way that produces no error if
the directory is already present, while still raising
FileExistsError if a non-directory filesystem entry (such as a
regular file) is present where we want the directory to be.

Author: EliahKagan

Diff for: gitdb/db/loose.py

@@ -8,7 +8,6 @@
     ObjectDBW
 )
 
-
 from gitdb.exc import (
     BadObject,
     AmbiguousObjectName
@@ -33,10 +32,8 @@
     bin_to_hex,
     exists,
     chmod,
-    isdir,
     isfile,
     remove,
-    mkdir,
     rename,
     dirname,
     basename,
@@ -222,8 +219,7 @@ def store(self, istream):
         if tmp_path:
             obj_path = self.db_path(self.object_path(hexsha))
             obj_dir = dirname(obj_path)
-            if not isdir(obj_dir):
-                mkdir(obj_dir)
+            os.makedirs(obj_dir, exist_ok=True)
             # END handle destination directory
             # rename onto existing doesn't work on NTFS
             if isfile(obj_path):