Should ignore X11/core by default

etckeeper should ignore /etc/X11/core:

bzr: warning: skipping /etc/X11/core (larger than add.maximum_file_size of 20000000 bytes)
Committing to: /etc/
bzr: ERROR: No changes to commit. Please 'bzr add' the files you want to commit, or use --unchanged to force an empty commit.
E: Problem executing scripts DPkg::Post-Invoke 'if [ -x /usr/bin/etckeeper ]; then etckeeper post-install; fi'
E: Sub-process returned an error code

$ ls -l /etc/X11/core -h
-rw------- 1 root root 21M Jul 22 18:26 /etc/X11/core