run-tests: check if stream is a tty before using color
Previous implementation (
e80041832eec) checked only if sys.stderr was a tty
which was less general. Also makes sure that colors is never used if pygments is
not available, irrespective of --color flag value.
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a