Bug#975455: devtodo: failing autopkgtests with grep 3.6

[Sven Joachim <svenjoac@gmx.de>]

Source: devtodo
Version: 0.1.20-8
Severity: serious
X-Debbugs-Cc: Carlos Henrique Lima Melara <charlesmelara@outlook.com>, Sven Joachim <svenjoac@gmx.de>

The purge-tasks and rm-tasks-and-db-file autopkgtests are currently
failing on ci.debian.net, creating a migration problem for ncurses.  It
seems however that this has nothing to do with the new ncurses version
but is related to a change in grep 3.5:

,----
|   The --files-without-match (-L) option has reverted to its behavior
|   in grep 3.1 and earlier.  That is, grep -L again succeeds when a
|   line is selected, not when a file is listed.  The behavior in grep
|   3.2 through 3.4 was causing compatibility problems.
`----

Both tests in question run grep -L as their last command and are failing
with grep 3.6-1 while they succeeded with earlier grep versions (prior
to 2020-11-17, testing had grep 3.4-1).