Restrict length of repo description on repolist page

If any repo has a very long description, all repos suffer since the
repo-links in the right-most column gets pushed out of sight.

Fix it by introducing max-repodesc-length parameter in cgitrc, and default
to 60 chars.

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

cgit.h

@@ -89,6 +89,7 @@ extern int cgit_cache_static_ttl;
 extern int cgit_cache_max_create_time;
 
 extern int cgit_max_msg_len;
+extern int cgit_max_repodesc_len;
 extern int cgit_max_commit_count;
 
 extern char *cgit_repo_name;