Hi,

I've noticed that `apt showsrc` doesn't seem to respect the -t=target_release option.
When running `apt -t nonexist showsrc somepackage`, it prints one, which
is not the expected behavior.

apt should either support the `target_release` option, or exit failure
without outputting an entry similar to `apt show`

	E: Command line option 't' [from -t] is not understood in combination with the other options.

This bug may relate to #613664 and #776366.

Best regards,
Tianyu Chen @ deepin