git bisect view's use of DISPLAY environment variable in Cygwin

Greetings,

git bisect view uses gitk if DISPLAY environment variable is set and git log otherwise. Since gitk doesn't require X server in Cygwin, that seems like a bit questionable condition in that environment.

I'd prefer it to use gitk unless an option given. I think an option would be preferable (to DISPLAY= git bisect view) in Unix as well if you have DISPLAY set but you want it to use git log.