Check if additional git provider is not the original git provider (#26533)

Release Notes:

- N/A

Yesterday I worked on https://github.com/zed-industries/zed/pull/26482
and noticed afterwards that we have duplicated hosting providers if the
git remote host is "gitlab.com" and after the PR also for "github.com".
This is not a big problem, since the original providers are registered
first and therefore we first find a match with the original providers,
but I think we should address this nevertheless.

We initialize every hosting provider with the defaults here:

https://github.com/zed-industries/zed/blob/b008b2863ee015a9dc6ecdcd6dedbc708983f8b3/crates/git_hosting_providers/src/git_hosting_providers.rs#L15-L24

After that, we also register additional hosting providers:

https://github.com/zed-industries/zed/blob/b008b2863ee015a9dc6ecdcd6dedbc708983f8b3/crates/git_hosting_providers/src/git_hosting_providers.rs#L30-L43

If we do not check if the additional provider is not the original
provider, we will register the same provider twice.

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Co-authored-by: Marshall Bowers <git@maxdeviant.com>