bisect: visualize with git-log if gitk is unavailable
If gitk is not available in the PATH, bisect ends up exiting with the shell's 127 error code, confusing the git wrapper into thinking that bisect is not a git command.
We already fallback to git-log if there doesn't seem to be a graphical display available. We should do the same if gitk is not available in our PATH at all. This not only fixes the ugly error message, but is a much more sensible default than failing to show the user anything.