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

[Hannu Koivisto <azure@iki.fi>]

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.

-- 
Hannu