"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>