Find the HASHTAGS file even if you are not at the checkout root.
The post upload hook was failing if you were not at the root, this fixes it.

NOTRY=true

Review URL: https://codereview.chromium.org/1058483002
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 8b60774..a036be6 100644
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -392,7 +392,8 @@
                 'Automatically added \'NOTRY=true\' to the CL\'s description'))
 
     # Read and process the HASHTAGS file.
-    with open('HASHTAGS', 'rb') as hashtags_csv:
+    hashtags_fullpath = os.path.join(change._local_root, 'HASHTAGS')
+    with open(hashtags_fullpath, 'rb') as hashtags_csv:
       hashtags_reader = csv.reader(hashtags_csv, delimiter=',')
       for row in hashtags_reader:
         if not row or row[0].startswith('#'):