ignore git when computing signature of a recipe develop egg

When a recipe is developed in a git working copy, .git administrative
database folder should be ignored, like we did with CVS and svn.

/reviewed-on !14

src/zc/buildout/buildout.py

@@ -1863,7 +1863,7 @@ def _open(base, filename, seen, dl_options, override, downloaded):
     return result
 
 
-ignore_directories = '.svn', 'CVS', '__pycache__'
+ignore_directories = '.svn', 'CVS', '__pycache__', '.git'
 _dir_hashes = {}
 def _dir_hash(dir):
     dir_hash = _dir_hashes.get(dir, None)