Add support for adding labels based on files changed

```toml
[autolabel."A-rustdoc"]
trigger_labels = [ ... ]
exclude_labels = [ ... ]
trigger_files = [
    "src/librustdoc",
    "src/tools/rustdoc",
    "src/rustdoc-json-types",
]
```

Author: Mark-Simulacrum

src/config.rs

@@ -117,6 +117,8 @@ pub(crate) struct AutolabelLabelConfig {
     pub(crate) trigger_labels: Vec<String>,
     #[serde(default)]
     pub(crate) exclude_labels: Vec<String>,
+    #[serde(default)]
+    pub(crate) trigger_files: Vec<String>,
 }
 
 #[derive(PartialEq, Eq, Debug, serde::Deserialize)]

src/github.rs

@@ -753,6 +753,52 @@ pub struct IssuesEvent {
     pub repository: Repository,
     /// Some if action is IssuesAction::Labeled, for example
     pub label: Option<Label>,
+
+    // These fields are the sha fields before/after a synchronize operation,
+    // used to compute the diff between these two commits.
+    #[serde(default)]
+    before: Option<String>,
+    #[serde(default)]
+    after: Option<String>,
+
+    #[serde(default)]
+    base: Option<CommitBase>,
+    #[serde(default)]
+    head: Option<CommitBase>,
+}
+
+#[derive(Default, Clone, Debug, serde::Deserialize)]
+pub struct CommitBase {
+    sha: String,
+}
+
+impl IssuesEvent {
+    /// Returns the diff in this event, for Open and Synchronize events for now.
+    pub async fn diff_between(&self, client: &GithubClient) -> anyhow::Result<Option<String>> {
+        let (before, after) = if self.action == IssuesAction::Synchronize {
+            (
+                self.before.clone().unwrap_or_default(),
+                self.after.clone().unwrap_or_default(),
+            )
+        } else if self.action == IssuesAction::Opened {
+            (
+                self.base.clone().unwrap_or_default().sha,
+                self.head.clone().unwrap_or_default().sha,
+            )
+        } else {
+            return Ok(None);
+        };
+
+        let mut req = client.get(&format!(
+            "{}/compare/{}...{}",
+            self.issue.repository().url(),
+            before,
+            after
+        ));
+        req = req.header("Accept", "application/vnd.github.v3.diff");
+        let diff = client.send_req(req).await?;
+        Ok(Some(String::from(String::from_utf8_lossy(&diff))))
+    }
 }
 
 #[derive(Debug, serde::Deserialize)]

src/handlers.rs

@@ -115,7 +115,7 @@ macro_rules! issue_handlers {
             errors: &mut Vec<HandlerError>,
         ) {
             $(
-            match $name::parse_input(ctx, event, config.$name.as_ref()) {
+            match $name::parse_input(ctx, event, config.$name.as_ref()).await {
                 Err(err) => errors.push(HandlerError::Message(err)),
                 Ok(Some(input)) => {
                     if let Some(config) = &config.$name {

src/handlers/autolabel.rs

@@ -7,11 +7,55 @@ pub(super) struct AutolabelInput {
     labels: Vec<Label>,
 }
 
-pub(super) fn parse_input(
-    _ctx: &Context,
+pub(super) async fn parse_input(
+    ctx: &Context,
     event: &IssuesEvent,
     config: Option<&AutolabelConfig>,
 ) -> Result<Option<AutolabelInput>, String> {
+    if let Some(diff) = event
+        .diff_between(&ctx.github)
+        .await
+        .map_err(|e| {
+            log::error!("failed to fetch diff: {:?}", e);
+        })
+        .unwrap_or_default()
+    {
+        if let Some(config) = config {
+            let mut files = Vec::new();
+            for line in diff.lines() {
+                // mostly copied from highfive
+                if line.starts_with("diff --git ") {
+                    let parts = line[line.find(" b/").unwrap() + " b/".len()..].split("/");
+                    let path = parts.collect::<Vec<_>>().join("/");
+                    if !path.is_empty() {
+                        files.push(path);
+                    }
+                }
+            }
+            let mut autolabels = Vec::new();
+            for trigger_file in files {
+                if trigger_file.is_empty() {
+                    // TODO: when would this be true?
+                    continue;
+                }
+                for (label, cfg) in config.labels.iter() {
+                    if cfg
+                        .trigger_files
+                        .iter()
+                        .any(|f| trigger_file.starts_with(f))
+                    {
+                        autolabels.push(Label {
+                            name: label.to_owned(),
+                        });
+                    }
+                }
+                if !autolabels.is_empty() {
+                    return Ok(Some(AutolabelInput { labels: autolabels }));
+                }
+            }
+        }
+    }
+
     if event.action == IssuesAction::Labeled {
         if let Some(config) = config {
             let mut autolabels = Vec::new();

src/handlers/major_change.rs

@@ -14,7 +14,7 @@ pub enum Invocation {
     Rename { prev_issue: ZulipGitHubReference },
 }
 
-pub(super) fn parse_input(
+pub(super) async fn parse_input(
     _ctx: &Context,
     event: &IssuesEvent,
     _config: Option<&MajorChangeConfig>,

src/handlers/notify_zulip.rs

@@ -20,7 +20,7 @@ pub(super) enum NotificationType {
     Reopened,
 }
 
-pub(super) fn parse_input(
+pub(super) async fn parse_input(
     _ctx: &Context,
     event: &IssuesEvent,
     config: Option<&NotifyZulipConfig>,