Run fetch before testing if master contains beta

Author: flip1995

doc/release.md

@@ -63,7 +63,14 @@ to the beta Rust release. The remerge is then necessary, to make sure that the
 Clippy commit, that was used by the now stable Rust release, persists in the
 tree of the Clippy repository.
 
-To find out if this step is necessary run `git branch master --contains beta`.
+To find out if this step is necessary run
+
+```bash
+# Assumes that the local master branch is up-to-date
+$ git fetch upstream
+$ git branch master --contains upstream/beta
+```
+
 If this command outputs `master`, this step is **not** necessary.
 
 ```bash