Make presubmit bark at FIXED= in CL description

"Fixed:" is equivalent and the preferred syntax.

Also sets up a more generic mechanism for "unwanted tags",
which BUG= and TBR= may eventually be when we're ready to
move over to git-footers more fully.

Bug: monorail:4470
Change-Id: I2e999d364292447124870b8d3539fde1ac1eb4ec
Recipe-Nontrivial-Roll: chromiumos
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1836607
Auto-Submit: Dan Beam <dbeam@chromium.org>
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Aaron Gable <agable@chromium.org>

presubmit_canned_checks.py

@@ -49,6 +49,22 @@ def CheckChangeHasBugField(input_api, output_api):
     return [output_api.PresubmitNotifyResult(
         'If this change has an associated bug, add Bug: [bug number].')]
 
+def CheckChangeHasNoUnwantedTags(input_api, output_api):
+  UNWANTED_TAGS = {
+      'FIXED': {
+          'why': 'is not supported',
+          'instead': 'Use "Fixed:" instead.'
+      },
+      # TODO: BUG, ISSUE
+  }
+
+  errors = []
+  for tag, desc in UNWANTED_TAGS.items():
+    if tag in input_api.change.tags:
+      subs = tag, desc['why'], desc.get('instead', '')
+      errors.append(('%s= %s. %s' % subs).rstrip())
+
+  return [output_api.PresubmitError('\n'.join(errors))] if errors else []
 
 def CheckDoNotSubmitInDescription(input_api, output_api):
   """Checks that the user didn't add 'DO NOT ''SUBMIT' to the CL description.

tests/presubmit_unittest.py

@@ -1582,6 +1582,11 @@ class CannedChecksUnittest(PresubmitTestsBase):
                          presubmit.OutputApi.PresubmitNotifyResult,
                          False)
 
+  def testCannedCheckChangeHasNoUnwantedTags(self):
+    self.DescriptionTest(presubmit_canned_checks.CheckChangeHasNoUnwantedTags,
+                         'Foo\n', 'Foo\nFIXED=1234',
+                         presubmit.OutputApi.PresubmitError, False)
+
   def testCheckChangeHasDescription(self):
     self.DescriptionTest(presubmit_canned_checks.CheckChangeHasDescription,
                          'Bleh', '',