Issue 2972 - Properly generate Git ignore paths for dependencies

ensure_dependencies.py

 #!/usr/bin/env python
 # coding: utf-8
 
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 import sys
 import os
 import posixpath
@@ skipping 108 matching lines @@
                            cwd=repo, stdout=devnull, stderr=devnull) == 0:
           newly_tracked = True
     # Finally fetch any newly tracked remote branches
     if newly_tracked:
       subprocess.check_call(["git", "fetch", "--quiet", "origin"], cwd=repo)
 
   def update(self, repo, rev):
     subprocess.check_call(["git", "checkout", "--quiet", rev], cwd=repo)
 
   def ignore(self, target, repo):
-    module = os.path.relpath(target, repo)
+    module = os.path.join(os.path.sep, os.path.relpath(target, repo))
     exclude_file = os.path.join(repo, ".git", "info", "exclude")
     _ensure_line_exists(exclude_file, module)
 
   def postprocess_url(self, url):
     # Handle alternative syntax of SSH URLS
     if "@" in url and ":" in url and not urlparse.urlsplit(url).scheme:
       return "ssh://" + url.replace(":", "/", 1)
     return url
 
 repo_types = OrderedDict((
@@ skipping 222 matching lines @@
   args = parser.parse_args()
 
   if args.quiet:
     logging.disable(logging.INFO)
 
   repos = args.repos
   if not len(repos):
     repos = [os.path.dirname(__file__)]
   for repo in repos:
     resolve_deps(repo)