config: use config_error_nonbool() instead of custom messages

A few config callbacks use their own custom messages to report an
unexpected implicit bool like:

  [merge "foo"]
  driver

These should just use config_error_nonbool(), so the user sees
consistent messages.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff-interface.c

@@ -314,7 +314,7 @@ int git_xmerge_config(const char *var, const char *value,
 {
 	if (!strcmp(var, "merge.conflictstyle")) {
 		if (!value)
-			return error(_("'%s' is not a boolean"), var);
+			return config_error_nonbool(var);
 		if (!strcmp(value, "diff3"))
 			git_xmerge_style = XDL_MERGE_DIFF3;
 		else if (!strcmp(value, "zdiff3"))