Tweak to make it clearer what to do if your working space is dirty

If you try to biect in a directory with .gitignore'd files in it
bisect will refuse to run. Since these files arent shown by status
this can cause some head scratching.

This patch makes it more obvious what is wrong by showing the listed
files and suggesting git clean as a solution. I deliberately did not
document the command to clean up untracked files, so people wouldn't
knee jerk paste it somewhere and lose something important. And by people
I mean me. :-) Thus it just shows the command to wipe ignored files,
which I figure is safe.

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