Disable colored output on a dumb terminal (#3552)

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.

src/main.c

@@ -543,7 +543,10 @@ int main(int argc, char* argv[]) {
 #ifdef USE_ISATTY
   if (isatty(STDOUT_FILENO)) {
 #ifndef WIN32
-    dumpopts |= JV_PRINT_ISATTY | JV_PRINT_COLOR;
+    dumpopts |= JV_PRINT_ISATTY;
+    char *term = getenv("TERM");
+    if (term != NULL && term[0] != '\0' && strcmp(term, "dumb"))
+      dumpopts |= JV_PRINT_COLOR;
 #else
   /* Verify we actually have the console, as the NUL device is also regarded as
      tty.  Windows can handle color if ANSICON (or ConEmu) is installed, or