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.
magit-cmd-output: do not corrupt output

This fixes #544.  There is no reason why we would want to manipulate
git's output.  In the master and next branches something similar to
the code removed in this commit was used to have colorful graphs in
logs.  But doing it in `magit-cmd-output' was wrong and that has been
fixed in these branches also.
magit-cmd-output: do not corrupt output

This fixes #544.  There is no reason why we would want to manipulate
git's output.  In the master and next branches something similar to
the code removed in this commit was used to have colorful graphs in
logs, but in maint that isn't the case.
Merge pull request #641 from tarsius/maint

magit-cmd-output: do not corrupt output
Merge branch 'tarsius/colour'
Merge branch 'maint'
Merge branch 'master' into next