Support only publishing files added to git

[DartBot]

<img src="https://avatars.githubusercontent.com/u/817422?v=3" align="left" width="96" height="96"hspace="10"> Issue by Pajn
Originally opened as dart-lang/sdk#23375

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When running pub publish it wants to publish all files inside the project, it would be great with a flag to only publish files that are added to the git repository.

This is to avoid publishing build files, work-in-progress, caches or other similar stuff.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/8631949?v=3" align="left" width="48" height="48"hspace="10"> Comment by floitschG

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Added Area-Pub, Triaged labels.

[natebosch]

Pub honors the .gitignore file now which solve the biggest part of this problem. It doesn't help for WIP type scenarios, but I don't think that's something we necessarily want to focus on.