discovery: move cl.hasnode outside of the for-loop IIUC, resolving attributes for changelog can lead to some overhead. So this patch moves that to outside of a for-loop. Differential Revision: https://phab.mercurial-scm.org/D6147

discovery: prevent deleting items from a dictionary Removing elements from Python dictionary is expensive. So let's prevent adding them instead. I added a newline to make code look a bit better. Differential Revision: https://phab.mercurial-scm.org/D6146