Auto merge of #17761 - RalfJung:josh-roudntrip-error, r=lnicola

when josh-proxy screws up the roundtrip, say what the involved commits are

Helps debugging rust-lang/rust-analyzer#17733

src/tools/rust-analyzer/xtask/src/release.rs

@@ -202,7 +202,10 @@ impl flags::RustcPush {
         let head = cmd!(sh, "git rev-parse HEAD").read()?;
         let fetch_head = cmd!(sh, "git rev-parse FETCH_HEAD").read()?;
         if head != fetch_head {
-            bail!("Josh created a non-roundtrip push! Do NOT merge this into rustc!");
+            bail!(
+                "Josh created a non-roundtrip push! Do NOT merge this into rustc!\n\
+                Expected {head}, got {fetch_head}."
+            );
         }
         println!("Confirmed that the push round-trips back to rust-analyzer properly. Please create a rustc PR:");
         // https://github.com/github-linguist/linguist/compare/master...octocat:linguist:master