GnuPG 2.2.19 added a warning when no command was given.

* src/password-store.sh (reencrypt_path): Add --decrypt to --list-only
* tests/t0300-reencryption.sh (gpg_keys_from_encrypted_file): same

https://bugs.gnupg.org/gnupg/msg9873
http://git.gnupg.org/cgi-bin/gitweb.cgi?p=gnupg.git;a=commit;h=810adfd47801fc01e45fb71af9f05c91f7890cdb
https://bugzilla.suse.com/show_bug.cgi?id=1028867

This reverts commit ec8140b0.

The needed option is only available on GnuPG 2.1, and we need to support
GnuPG 2.0 too.

This is required by most graphical editors, as the shell command they
provide needs to be told to wait until the file has been closed.

This reverts '311c98c9'. We thus use
PATH so that the tests still pass in directories with spaces.

Key discovery parses stderr output of gnupg. The secmem warning
interferes with parsing.