Prompt user if tag already exists (instead of requiring --force or manual cleanup)

[fingolfin]

When there is a problem while running release, it often happen that the version tag (say, v1.2.3) was already created. If one then has to change history to resolve the problems (by adding commits, or modifying existing commits), the tag doesn't point at HEAD anymore, which causes release to abort with an error.

So to resume the release process, one needs to either use --force or manually delete or move the bad tag.

Instead, we could show a prompt to the user, something like "the tag v1.2.3 already exists on commit != HEAD. Shall I force update it [y/N]". If you just press return, it aborts.

[olexandr-konovalov]

I came across this and was deleting the tag manually. Good idea to make this more user-friendly.