ui-shared: use placeholder for empty commit subject

Display a placeholder when creating links to commits with empty subjects.
This avoids the creation of links of the form <a></a>.
diff --git a/ui-shared.c b/ui-shared.c
index d9d5da7..75b97a1 100644
--- a/ui-shared.c
+++ b/ui-shared.c
@@ -361,7 +361,10 @@
 		delim = "&amp;";
 	}
 	html("'>");
-	html_txt(name);
+	if (name[0] != '\0')
+		html_txt(name);
+	else
+		html_txt("(no commit message)");
 	html("</a>");
 }