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-snapshot.c

@@ -19,7 +19,7 @@ static int write_compressed_tar_archive(struct archiver_args *args,const char *f
 	f.argv = malloc(2 * sizeof(char *));
 	f.argv[0] = f.cmd;
 	f.argv[1] = NULL;
-	cgit_open_filter(&f, NULL);
+	cgit_open_filter(&f);
 	rv = write_tar_archive(args);
 	cgit_close_filter(&f);
 	return rv;