Adjust RFC rendered linkifier.

This changes the URL used in the RFC "Rendered" link to link directly to
the author's branch instead of a specific commit. The problem with
specific commits is that they need to be updated on every push. The
existing code didn't handle that (it ignored if it already had a link),
and I think SHA blob links are less desirable since they have a risk
that reviewers will be looking at outdated content.

This also drops the check for `Synchronize` events, since this isn't
updating the rendered link on every push.

src/github.rs

@@ -284,10 +284,10 @@ pub struct Issue {
 
     /// The base commit for a PR (the branch of the destination repo).
     #[serde(default)]
-    base: Option<CommitBase>,
+    pub base: Option<CommitBase>,
     /// The head commit for a PR (the branch from the source repo).
     #[serde(default)]
-    head: Option<CommitBase>,
+    pub head: Option<CommitBase>,
 }
 
 /// Contains only the parts of `Issue` that are needed for turning the issue title into a Zulip
@@ -904,6 +904,9 @@ struct PullRequestEventFields {}
 #[derive(Clone, Debug, serde::Deserialize)]
 pub struct CommitBase {
     sha: String,
+    #[serde(rename = "ref")]
+    pub git_ref: String,
+    pub repo: Repository,
 }
 
 pub fn files_changed(diff: &str) -> Vec<&str> {

src/handlers/rfc_helper.rs

@@ -23,15 +23,29 @@ pub async fn handle(ctx: &Context, event: &Event) -> anyhow::Result<()> {
 }
 
 async fn add_rendered_link(ctx: &Context, e: &IssuesEvent) -> anyhow::Result<()> {
-    if e.action == IssuesAction::Opened || e.action == IssuesAction::Synchronize {
+    if e.action == IssuesAction::Opened {
         let files = e.issue.files(&ctx.github).await?;
 
         if let Some(file) = files.iter().find(|f| f.filename.starts_with("text/")) {
             if !e.issue.body.contains("[Rendered]") {
+                // This URL should be stable while the PR is open, even if the
+                // user pushes new commits.
+                //
+                // It will go away if the user deletes their branch, or if
+                // they reset it (such as if they created a PR from master).
+                // That should usually only happen after the PR is closed.
+                // During the closing process, the closer should update the
+                // Rendered link to the new location (which we should
+                // automate!).
+                let head = e.issue.head.as_ref().unwrap();
+                let url = format!(
+                    "https://github.com/{}/blob/{}/{}",
+                    head.repo.full_name, head.git_ref, file.filename
+                );
                 e.issue
                     .edit_body(
                         &ctx.github,
-                        &format!("{}\n\n[Rendered]({})", e.issue.body, file.blob_url),
+                        &format!("{}\n\n[Rendered]({})", e.issue.body, url),
                     )
                     .await?;
             }