Fix: don't try to copy non-existing files

[jankaifer]

When trying to create a new file with agenix -e It printed a weird cp error, that was confusing, and that hid the actual error I faced at #211.

Fixes #211

[jankaifer]

Tested by running this:

$ echo a | agenix -e login-password.age
$ echo a | agenix -e login-password.age
login-password.age wasn't changed, skipping re-encryption.

First doesn't print that error anymore and that diffing logic was not broken.

[CobaltCause]

Duplicate of #176?