As of git 2.7, you can use the arguments –term-old and –term-new.
For instance, you can identify a problem-fixing commit thus:
git bisect start --term-new=fixed --term-old=unfixed
git bisect fixed master
git bisect unfixed $some-old-sha1
As you test, say git bisect fixed or git bisect unfixed as appropriate.
Old answer, for versions of git prior to 2.7
Instead of temporarily training yourself to think that bad means good and good means bad, why not create some aliases?
In ~/.gitconfig add the following:
[alias]
bisect-fixed = bisect bad
bisect-unfixed = bisect good
You can start identifying a problem-fixing commit thus:
$ git bisect start
$ git bisect-fixed master
$ git bisect-unfixed $some-old-sha1
As you test, say git bisect-fixed or git bisect-unfixed as appropriate.