Enable colored output by default only when the TERM environment variable
is set to a value other than dumb, as git and grep do, so that users no
longer need to pass -M or set NO_COLOR on terminals that cannot display
escape sequences. Colored output is now disabled not only for TERM=dumb
but also when TERM is unset or empty. The -C option still enables colors
in all these cases. On Windows, colored output is still decided by
ANSICON and the console mode, and TERM is not checked.
