lei convert: mail format conversion sub-command

lei convert: mail format conversion sub-command

This will make testing IMAP support for other commands easier, as
it doesn't write to lei/store at all.  Like the pager and MUA,
"git credential" is always spawned by script/lei (and not
lei-daemon) so it has a controlling terminal for password
prompts.

v2: fix missing requires, correct test ordering
v3: ensure config exists for IMAP auth