Add a flag to skip files that already have REUSE/SPDX tags

[silverhook]

It would make sense to also have a --skip-tagged flag to addheader to skip files that already include SPDX/REUSE file tags/headers.

There are two ways how I see we could implement this:

1. skip file as soon as you find any SPDX- tag – risky IMHO, especially if there are non-REUSE tags or Snippet tags.
2. skip file only if both SPDX-License-Identifier and SPDX-FileCopyrightText are present – my preference.

[silverhook]

I see--skip-existing flag already does that, but it seems it does the option 1. above.

Could we instead change it to do option 2. instead?

[github-actions[bot]]

Thank you for your time and contributions! Unfortunately, this issue has been inactive for quite a while which means we probably can't manage the time to deal with it. That's why we're marking it as stale. We want to keep things tidy and focus on active discussions, but we’re always happy to revisit if this is still relevant!

If you’d like to keep this open, please add a comment to let us know and remove the label stale. Otherwise, this issue will most likely be automatically closed soon.

[silverhook]

This is still relevant IMHO.

[carmenbianca]

Hi! This repository is being migrated away from GitHub towards Codeberg. More context can be found in #865. This issue can henceforth be found at https://codeberg.org/fsfe/reuse-tool/issues/620. Any comments subsequent to this comment will not be migrated.