allow sharing via github repositories

[nt4f04uNd]

at the moment, to share the dart pad we need to create a gist for it

i propose that we could also host a dart pad from any other git resourse, for example i have a repository which contains examples and reproducibles for all kinds of issues i need, to not clutter my github account

i would like to be able to specify a url to github ref (for example https://github.com/nt4f04unds-archive/issues/tree/flutter_71342) and host the pad like that

[RedBrogdon]

Thanks for the feature request, @nt4f04uNd.

This is something we'd like to get done. We're tracking this feature request at #1769, so I'm going to close this issue as a duplicate.