--benchmark_color=auto (the default value) always prints color

[guoyr]

Benchmark defaults to "auto" for deciding to whether to use color or not, as part of the following conditional:

  if ((FLAGS_benchmark_color == "auto" && IsColorTerminal()) ||
      IsTruthyFlagValue(FLAGS_benchmark_color)) {
    // print color.
}

But IsTruthyFlagValue is permissive and returns true for the string "auto", causing IsColorTerminal() to be ignored.

[dmah42]

PR in progress.