checkpatch.pl: false warning about "-p0 patch"

[Brian Norris <computersforpeace@gmail.com>]

Hello,

I have received what appears to be a false warning from checkpatch.pl when
checking a patch named 'b':

    $ scripts/checkpatch.pl b
    WARNING: patch prefix 'b' exists, appears to be a -p0 patch

    total: 0 errors, 1 warnings, 16 lines checked

    b has style problems, please review.

    If any of these errors are false positives, please report
    them to the maintainer, see CHECKPATCH in MAINTAINERS.

It appears that I can trigger this warning with any arbitrary commit. The
following bash script gives me at least one warning for everything in Linus'
tree:

  for i in {0..100}
  do
      git format-patch -1 HEAD~$i --stdout > b
      scripts/checkpatch.pl b
  done

I have also seen this warning on patches with other names, but I'm not
reproducing that right now.

Brian