Auto Update Mode Version Suffix not working correctly

The suffix feature for auto update mode mentioned here is broken right now. I noticed this yesterday with this commit: fdroiddata@9ac48a58...35c36c93

There are multiple issues, some of them are, I think, inherent to the current design of the update check:

1. The + at the beginning of the suffix string is not stripped out. --> trivial fix
2. The tag name in the commit field gets set to the version + suffix. This makes no sense because if I wanted that behavior I could have chosen a different auto update pattern without the version suffix. --> Set the commit to just the CV without the suffix.
3. The CV is set without the suffix during an update check.

• This is tricky because this essentially references the upstream version of the application without the fdroid specific suffix and is needed for setting the correct commit(tag)(see 2.).
• At the same time this field is used for the recommended version (marketversion) in the index file later which is wrong here because the suffix is missing.

Nevertheless I think this is a feature that should be working so I would propose the following changes:

• Add the version suffix as an explicit attribute to the application.
• Let the CV field reference the upstream versions.
• When building the index file add the application suffix to the marketversion tag.

I'm not sure how best to do the first. Probably cleanest to add a new field optional field in the metadata. I can prepare a MR if there is a consensus.