Update file_url() handling of Thunderbird comm paths

In commit df291c6, root_dir was
changed from a relative path to an absolute path, resulting in
file_url() no longer detecting the path correctly as it didn't
start with "comm/". This change allows file_url() to correctly
detect and handle an absolute path that includes "comm/".

Author: coreycb

src/taskgraph/parameters.py

@@ -253,8 +253,8 @@ def file_url(self, path, pretty=False):
         :return str: The URL displaying the given path.
         """
         if self["repository_type"] == "hg":
-            if path.startswith("comm/"):
-                path = path[len("comm/") :]
+            if "comm/" in path:
+                path = path.split("comm/")[1]
                 repo = self["comm_head_repository"]
                 rev = self["comm_head_rev"]
             else: