Bug 1380118 - aom: Record the upstream repo we vendored from. r=froydnj

We've been recording the commit id from the last vendor in
README_MOZILLA inside the various media directories. Since
we now support a --repo switch to pull from forks, record
this info as well, to make it easier to find contiguous
upstream source.

MozReview-Commit-ID: 1RanpkWfAeC

--HG--
extra : rebase_source : b6bd16b56626a871802822385be6f3a24db6cd50

media/libaom/README_MOZILLA

@@ -14,8 +14,12 @@ To update to a specific upstream git tag or commit, use
 
   ./mach vendor aom -r <commit>
 
-The upstream aom git repository is:
+The upstream git repository is https://aomedia.googlesource.com/aom
 
-    https://aomedia.googlesource.com/aom
+To update to a fork, use
+
+  ./mach vendor aom --repo <repository url> [-r <commit>]
+
+The last update was pulled from https://aomedia.googlesource.com/aom
 
 The git commit ID used was f5bdeac22930ff4c6b219be49c843db35970b918 (Thu Jul 27 18:23:37 2017 +0000).

python/mozbuild/mozbuild/vendor_aom.py

@@ -126,6 +126,11 @@ Please set a repository url with --repo on either googlesource or github.''' % h
         else:
             new_readme = '%s\n\n%s %s.' % (readme, prefix, revision)
 
+        prefix = 'The last update was pulled from'
+        new_readme = re.sub(prefix + ' https*://.*',
+                            prefix + ' %s' % self.repo_url,
+                            new_readme)
+
         if readme != new_readme:
             with open(filename, 'w') as f:
                 f.write(new_readme)