Implemented configurable HEAD shortlog on summary page.

This mirrors similiar functionality in gitweb. After clicking on
project on projectlist you will immediatelly see quick summary
of last N commits on HEAD.

[lh: changed from HEAD to cgit_query_head]

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

cgit.h

@@ -127,6 +127,7 @@ extern int cgit_cache_repo_ttl;
 extern int cgit_cache_dynamic_ttl;
 extern int cgit_cache_static_ttl;
 extern int cgit_cache_max_create_time;
+extern int cgit_summary_log;
 
 extern int cgit_max_msg_len;
 extern int cgit_max_repodesc_len;