Expose 'recursion_limit' option for merges

[wfchandler]

libgit2 has a recursion_limit option in git_merge_options to control the potential cost of recursive merging, but this is not currently exposed in rugged.

This change makes recursion_limit available for use in rugged.

[carlosmn]

Thanks. This would also need to be added to the list of supported options in the documentation for rb_git_tree_merge a bit further down the file.