Switch to exclusively using global ctx

Drop the context parameter from the following functions (and all static
helpers used by them) and use the global context instead:

* cgit_print_http_headers()
* cgit_print_docstart()
* cgit_print_pageheader()

Remove context parameter from all commands

Drop the context parameter from the following functions (and all static
helpers used by them) and use the global context instead:

* cgit_get_cmd()
* All cgit command functions.
* cgit_clone_info()
* cgit_clone_objects()
* cgit_clone_head()
* cgit_print_plain()
* cgit_show_stats()

In initialization routines, use the global context variable instead of
passing a pointer around locally.

Remove callback data parameter for cache slots

This is no longer needed since the context is always read from the
global context variable.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

ui-diff.c

@@ -407,7 +407,7 @@ void cgit_print_diff(const char *new_rev, const char *old_rev,
 		diff_setup_done(&diffopt);
 
 		ctx.page.mimetype = "text/plain";
-		cgit_print_http_headers(&ctx);
+		cgit_print_http_headers();
 		if (old_tree_sha1) {
 			diff_tree_sha1(old_tree_sha1, new_tree_sha1, "",
 				       &diffopt);