Show error on Fixes: and suggest Fixed:

The Bug: syntax is well known as a way of tagging bugs in a commit
message, at least partially because when it is missing there is a
presubmit warning suggesting that it be used.

The Fixed: syntax is less well known because there is no such reminder.
And, if somebody types "Fixes: " instead of "Fixed: " there is no error.
Therefore it is not surprising that "Fixes: " was used seven times in
the last 10,000 commits. See crrev.com/c/4416218 for one example.

This change adds both a reminder that Fixed: exists as well as an error
if Fixes: is used.

Fixed: 1434702
Change-Id: Id940d0af5cfea206ee923fbb779bf686b34c5f33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4436316
Reviewed-by: Aravind Vasudevan <aravindvasudev@google.com>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>

presubmit_canned_checks.py

@@ -71,16 +71,24 @@ _CORP_LINK_KEYWORD = '.corp.google'
 def CheckChangeHasBugField(input_api, output_api):
   """Requires that the changelist have a Bug: field."""
   bugs = input_api.change.BugsFromDescription()
+  results = []
   if bugs:
     if any(b.startswith('b/') for b in bugs):
-      return [
+      results.append(
           output_api.PresubmitNotifyResult(
-              'Buganizer bugs should be prefixed with b:, not b/.')
-      ]
-    return []
+              'Buganizer bugs should be prefixed with b:, not b/.'))
+  else:
+    results.append(
+        output_api.PresubmitNotifyResult(
+            'If this change has an associated bug, add Bug: [bug number] or '
+            'Fixed: [bug number].'))
+
+  if 'Fixes' in input_api.change.GitFootersFromDescription():
+    results.append(
+        output_api.PresubmitError(
+            'Fixes: is the wrong footer tag, use Fixed: instead.'))
+  return results
 
-  return [output_api.PresubmitNotifyResult(
-      'If this change has an associated bug, add Bug: [bug number].')]
 
 def CheckChangeHasNoUnwantedTags(input_api, output_api):
   UNWANTED_TAGS = {