only translate ansi sequences to properties when needed

Only translate ansi control sequences to text properties when colored
git output was explicitly requested.  Previously this was done for all
git output and as a result e.g. `magit-show's output was corrupted
when the shown file contained such sequences.  Fixes #544.

As I said in #620 this fixes the issue in `master` and `next` and I will merge into these branches tonight if nobody speaks up by then.