Support fetch before delete

[dermojo]

It would be handy to also support fetching before deleting a branch. Currently this is only supported for start and finish.
Git may complain if you try to delete a branch that's not fully merged, and if the merge happened remotely (e.g. in GitLab/GitHub/...), it doesn't have this information.

[alexrinass]

We're working on this. The implementation will add --fetch / --no-fetch flags to the delete command, following the same pattern used by start and finish. It will also support a gitflow.<type>.delete.fetch config key for persistent defaults.

When --fetch is used, the command fetches from the remote before deletion so Git can recognize remotely-merged branches — no more needing --force just because the merge happened on GitHub/GitLab. If no remote is configured, the fetch is silently skipped.