Make imgdiff work better with git:

- Suppress progress output if stderr is not a terminal (otherwise git
  diff spawns less that shows one very long line full of ^Ms).

- Make it possible to abort --highligh/--smart-highlight logic by pressing ^C
  (unfortunately this also aborts git diff, so if you're comparing many
  files and cancel -H for one of them with ^C, you won't get to see the
  remaining diffs).

- Abort --highlight/--smart-highlight logic if it takes longer than 10 seconds
  (timeout changeable with --timeout; use 0 to turn it off).