index: split up nt_init() in two I'd like to make nt_init() take a pointer to a nodetree to initialize, but it currently also allocates the nodetree. This patch prepares for that change by making nt_init() be about initializing an existing node tree and by creating a new index_init_nt() that creates the nodetree. Differential Revision: https://phab.mercurial-scm.org/D4114