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>

cgit.h

@@ -319,9 +319,11 @@ extern const char *cgit_repobasename(const char *reponame);
 
 extern int cgit_parse_snapshots_mask(const char *str);
 
-extern int cgit_open_filter(struct cgit_filter *filter, struct cgit_repo * repo);
+extern int cgit_open_filter(struct cgit_filter *filter);
 extern int cgit_close_filter(struct cgit_filter *filter);
 
+extern void cgit_prepare_repo_env(struct cgit_repo * repo);
+
 extern int readfile(const char *path, char **buf, size_t *size);
 
 extern char *expand_macros(const char *txt);