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>
diff --git a/ui-tree.c b/ui-tree.c
index 2d8d2f3..442b6be 100644
--- a/ui-tree.c
+++ b/ui-tree.c
@@ -45,7 +45,7 @@
 	if (ctx.repo->source_filter) {
 		html("<td class='lines'><pre><code>");
 		ctx.repo->source_filter->argv[1] = xstrdup(name);
-		cgit_open_filter(ctx.repo->source_filter, ctx.repo);
+		cgit_open_filter(ctx.repo->source_filter);
 		html_raw(buf, size);
 		cgit_close_filter(ctx.repo->source_filter);
 		free(ctx.repo->source_filter->argv[1]);