feat: add git-delete-gone-branches

[mkindahl]

When using GitHub and using the "Delete" button after merging a branch into a upstreams repository, the remote branch is gone but the local branch stays.

This command removes all "gone" branches that are still checked out.

I have tested this on macOs and Linux, but nothing else.

[mkindahl]

I get the following when running check_integrity.sh despite having added a line similar to the one for git-delete-merged-branches.

mats@fury:~/repos/git-extras$ bash check_integrity.sh 
Add description of git-delete-gone-branches in Commands.md

[hyperupcall]

Thank you for your contribution.

One main issue I see, is that if the branch includes the string "gone", it will automatically be removed, even if it's not actually gone from the remote.

Interestingly, we received a similar PR in #1239. I like the general approach that was decided on, but I'm not sure I'll be giving my approval for the pull request, for some unrelated reasons. But I don't want to ask you to change code if I'm not completely sure that PR #1239 isn't going to be merged.

[mkindahl]

Thank you for your contribution.

One main issue I see, is that if the branch includes the string "gone", it will automatically be removed, even if it's not actually gone from the remote.

Interestingly, we received a similar PR in #1239. I like the general approach that was decided on, but I'm not sure I'll be giving my approval for the pull request, for some unrelated reasons. But I don't want to ask you to change code if I'm not completely sure that PR #1239 isn't going to be merged.

The approach in #1239 does indeed look like a more solid approach. It might make sense to wait until it is closed before closing this PR, but feel free to close this PR once you know the situation.