Add support for config param summary-branches

This parameter can be used to specify max number of branches to show
on the summary page (if not all branches will be displayed, the "most
idle" branches are the ones to be pruned). The default value for this
parameter is 0, which disables the pruning.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -144,6 +144,7 @@ extern int cgit_cache_static_ttl;
 extern int cgit_cache_max_create_time;
 extern int cgit_summary_log;
 extern int cgit_summary_tags;
+extern int cgit_summary_branches;
 
 extern int cgit_max_msg_len;
 extern int cgit_max_repodesc_len;