This is effectively a revert of
commit 6812136bf5
Author: Jani Nikula <jani@nikula.org>
Date: Mon Mar 31 00:21:48 2014 +0300
lib: drop support for single-message mbox files
The intention was to drop support for indexing new single-message mbox
files (and whether that was a good idea in the first place is
arguable). However this inadvertently broke support for reading
headers from previously indexed single-message mbox files, which is
far worse.
Distinguishing between the two cases would require more code than
simply bringing back support for single-message mbox files.
0cc0144875