mail: split out the SMTP login to allow the keyring extension to wrap it The keyring extension only needs to tweak this tiny section of the larger function. But without any place to intercept the username/password fetching, it copy/pasted the entire function, and has grown a bunch of compatibility hacks to support older versions of Mercurial as well. Differential Revision: https://phab.mercurial-scm.org/D10471