cex: don't display twice unifying examples if there is no color

It makes no sense, and is actually confusing, to display twice the
same example with no visible difference.

* src/complain.h, src/complain.c (is_styled): New.
* src/counterexample.c (print_counterexample): Display the unified
example a second time only if it makes a difference.
* tests/conflicts.at, tests/counterexample.at, tests/report.at: Adjust.
* tests/diagnostics.at: Make sure we do display the unifying examples
twice when colors are enabled.  And check those colors.

src/complain.c

@@ -104,6 +104,23 @@ flush (FILE *out)
   fflush (out);
 }
 
+bool
+is_styled (FILE *out)
+{
+  if (out != stderr)
+    return false;
+  if (color_debug)
+    return true;
+#if HAVE_LIBTEXTSTYLE
+  return (color_mode == color_yes
+          || color_mode == color_html
+          || (color_mode == color_tty && isatty (STDERR_FILENO)));
+#else
+  return false;
+#endif
+}
+
+
 /*------------------------.
 | --warnings's handling.  |
 `------------------------*/
@@ -288,9 +305,7 @@ void
 complain_init_color (void)
 {
 #if HAVE_LIBTEXTSTYLE
-  if (color_mode == color_yes
-      || color_mode == color_html
-      || (color_mode == color_tty && isatty (STDERR_FILENO)))
+  if (is_styled (stderr))
     {
       style_file_prepare ("BISON_STYLE", "BISON_STYLEDIR", pkgdatadir (),
                           "bison-default.css");

src/complain.h

@@ -36,6 +36,8 @@ void end_use_class (const char *style, FILE *out);
 /** Flush \a out.  */
 void flush (FILE *out);
 
+/** Whether there's styling on OUT.  */
+bool is_styled (FILE *out);
 
 /*-------------.
 | --warnings.  |

src/counterexample.c

@@ -27,6 +27,7 @@
 #include <gl_rbtreehash_list.h>
 #include <hash.h>
 #include <stdlib.h>
+#include <textstyle.h>
 #include <time.h>
 
 #include "closure.h"
@@ -109,9 +110,15 @@ print_counterexample (counterexample *cex, FILE *out, const char *prefix)
            prefix, _("First derivation"));
   derivation_print (cex->d1, out, prefix);
 
-  fprintf (out, "  %s%-20s ",
-           prefix, cex->unifying ? _("Example") : _("Second example"));
-  derivation_print_leaves (cex->d2, out, prefix);
+  // If we output to the terminal (via stderr) and we have color
+  // support, display unifying examples a second time, as color allows
+  // to see the differences.
+  if (!cex->unifying || is_styled (stderr))
+    {
+      fprintf (out, "  %s%-20s ",
+               prefix, cex->unifying ? _("Example") : _("Second example"));
+      derivation_print_leaves (cex->d2, out, prefix);
+    }
   fprintf (out, "  %s%-20s ",
            prefix, _("Second derivation"));
   derivation_print (cex->d2, out, prefix);