feat: log repo and commit to admin log

They're in the build log too, and come from the trigger message, but
it's helpful for debugging to log them clearly.

Signed-off-by: Lars Wirzenius <liw@liw.fi>

src/engine.rs

@@ -105,6 +105,8 @@ impl Engine {
             } => {
                 let repo = req.repo();
                 let commit = req.commit();
+                self.adminlog
+                    .writeln(&format!("run CI for {repo} commit {commit}"))?;
 
                 match self.run_helper(repo, name, req.clone(), commit, None, Some((*id, title))) {
                     Ok(true) => success = true,