refactor AsyncTags to use AsyncJob

[extrawurst]

we have a lot of code duplication in the async implementations in the root of asyncgit/src/*. for a while we now have a unified abstraction that can be used instead: AsyncJob this is used already in AsyncRemoteTagsJob and AsyncSyntaxJob.

Lets one-by-one cleanup the other places starting with AsyncTags

[mvaneijk]

I would like to give this a try. It will be my first contribution to a rust project.

[extrawurst]

@mvaneijk may I suggest to start with asyncgit/src/tags.rs? It is essentially the same change but much less code, so probably a less daunting entry

[extrawurst]

@mvaneijk I added a more elaborate description above. hope that gives you a good place to start 👍

[mvaneijk]

@extrawurst Looks good, I'll give it a go.

[stale]

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