fix: github: restarting server creates duplicate webhook for repo

The Github routine for registering a new webhook, wasn't removing any
existing matching webhooks. There is a test for this, but it doesn't
assert that the delete requests are made. (This is a limitation of
kxio).

Closes kemitix/git-next#102

crates/forge-github/src/webhook/register.rs

@@ -1,5 +1,5 @@
 //
-use crate as github;
+use crate::{self as github, webhook};
 use git_next_config as config;
 use git_next_git as git;
 
@@ -15,8 +15,13 @@ pub async fn register(
         return Err(git::forge::webhook::Error::NoRepoConfig);
     };
 
-    let net = &github.net;
+    // remove any lingering webhooks for the same URL
+    let existing_webhook_ids = webhook::list(github, webhook_url).await?;
+    for webhook_id in existing_webhook_ids {
+        webhook::unregister(github, &webhook_id).await?;
+    }
 
+    let net = &github.net;
     let hostname = repo_details.forge.hostname();
     let authorisation = config::WebhookAuth::generate();
     let request = network::NetRequest::new(