upgrade-zulip-from-git: Run git fetch with --prune.

This prevents upgrading to an obsolete version of a branch that has
been deleted or renamed.

Signed-off-by: Anders Kaseorg <anders@zulip.com>
(cherry picked from commit 02582c6956)

scripts/lib/upgrade-zulip-from-git

@@ -81,7 +81,9 @@ try:
         )
 
     logging.info("Fetching the latest commits")
-    subprocess.check_call(["git", "fetch", "-q", "--tags", "--all"], preexec_fn=su_to_zulip)
+    subprocess.check_call(
+        ["git", "fetch", "--prune", "--quiet", "--tags", "--all"], preexec_fn=su_to_zulip
+    )
 
     # Generate the deployment directory via git worktree from our local repository.
     subprocess.check_call(