fix: show triggering commit in provider PR titles (#210)

This is relatively minor, but it's been bothering me for a while that
our individual providers often end up with changelogs like:

> • deps: Updated
([#549](cdktf/cdktf-provider-snowflake#549))
([df79e2e](cdktf/cdktf-provider-snowflake@df79e2e))
• deps: Updated
([#556](cdktf/cdktf-provider-snowflake#556))
([ab6c52d](cdktf/cdktf-provider-snowflake@ab6c52d))
• deps: Updated
([#558](cdktf/cdktf-provider-snowflake#558))
([28913f7](cdktf/cdktf-provider-snowflake@28913f7))
• deps: Updated
([#559](cdktf/cdktf-provider-snowflake#559))
([7f858c3](cdktf/cdktf-provider-snowflake@7f858c3))
• deps: Updated
([#560](cdktf/cdktf-provider-snowflake#560))
([899a086](cdktf/cdktf-provider-snowflake@899a086))
• deps: Updated
([#577](cdktf/cdktf-provider-snowflake#577))
([f9654e6](cdktf/cdktf-provider-snowflake@f9654e6))

Like, updated _what_? These messages aren't very useful.

These PR titles are generated when we push to main in this repo and the
changes don't result in CDKTF, JSII, the underlying provider, Node, or
Constructs being updated. I recognize that makes it difficult to
articulate what the actual update was. (Most often it's just projen.) I
figure at the very least as a fallback we can link back to the commit to
main in this repo to show what triggered the update in the first place.

With this change, the changelogs will now look like:

> • deps: Updated by 658ad9d
([#560](cdktf/cdktf-provider-snowflake#560))
([899a086](cdktf/cdktf-provider-snowflake@899a086))
• deps: Updated by a8236d4
([#577](cdktf/cdktf-provider-snowflake#577))
([f9654e6](cdktf/cdktf-provider-snowflake@f9654e6))

.github/lib/collect-changes.js

@@ -157,12 +157,29 @@ module.exports = async ({ core, exec }) => {
     );
   }
 
+  if (commitMessageParts.length === 0) {
+    const repo = 'cdktf/cdktf-repository-manager'; // we could make this dynamic
+    let commitHash;
+    try {
+      commitHash = (
+        await exec.getExecOutput("git", ["rev-parse", "--short", "HEAD"], {
+          cwd: path.join(process.env.GITHUB_WORKSPACE, "main"),
+        })
+      ).stdout;
+    } catch (e) {
+      console.log(e);
+    }
+    if (commitHash) {
+      commitMessageParts.push(`by ${repo}@${commitHash}`);
+    }
+  }
+
   if (hasBreakingChanges) {
     core.setOutput("has_breaking_changes", true);
   }
 
   core.setOutput(
     "commit_message",
-    `${prefix} ${commitMessageParts.join(", ")}`
+    `${prefix} ${commitMessageParts.join(", ")}`.trim()
   );
 };