cgit.c: always setup cgit repo environment variables

When cgit learned to setup environment variables for certain repo
settings before invoking a filter process, the setup occurred inside
cgit_open_filter().

This patch moves the setup out of cgit_open_filter() and into
prepare_repo_cmd() to prepare for additional uses of these variables.

Reviewed-by: Ferry Huberts <mailings@hupie.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-repolist.c

@@ -300,7 +300,7 @@ void cgit_print_site_readme()
 	if (!ctx.cfg.root_readme)
 		return;
 	if (ctx.cfg.about_filter)
-		cgit_open_filter(ctx.cfg.about_filter, NULL);
+		cgit_open_filter(ctx.cfg.about_filter);
 	html_include(ctx.cfg.root_readme);
 	if (ctx.cfg.about_filter)
 		cgit_close_filter(ctx.cfg.about_filter);