owners.py: partial fix for owners-check perf regression

owners.py

Index: owners.py
diff --git a/owners.py b/owners.py
index 78c7cca9cb7e6d9385fd954ce3512ce13ffd2da6..7773dbc38b6158005dc696cd14dc8c0ebb64bf8e 100644
--- a/owners.py
+++ b/owners.py
@@ -189,9 +189,11 @@ class Database(object):
 
   def load_data_needed_for(self, files):
     for f in files:
+      loaded_dirs = set()
       dirpath = self.os_path.dirname(f)
-      while not self._owners_for(dirpath):
+      while dirpath not in loaded_dirs:

[dtu, 2016/08/31 02:35:46]

Is loaded_dirs supposed to be at the top-level? I think as written it runs until
loaded_dirs becomes '', so this is the same as doing "while dirpath:"

"while dirpath:" would still be correct here. If it's faster than the current
check, then that's an easy change to make for both this function and
_enclosing_dir_with_owners.

         self._read_owners(self.os_path.join(dirpath, 'OWNERS'))
+        loaded_dirs.add(dirpath)
         if self._should_stop_looking(dirpath):
           break
         dirpath = self.os_path.dirname(dirpath)