backup: Set cwd while running pg_dump.

This avoids a spurious permission error inside the Postgres
`resolve_symlinks` function if we don’t have access to the current
working directory (e.g. we’re running with cwd /root inside `su
zulip`).

While we’re here, add a defensive `--` argument.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

zerver/management/commands/backup.py

@@ -64,7 +64,10 @@ class Command(ZulipBaseCommand):
 
             db_name = settings.DATABASES["default"]["NAME"]
             db_dir = os.path.join(tmp, "zulip-backup", "database")
-            run(["pg_dump", "--format=directory", db_name, "--file", db_dir])
+            run(
+                ["pg_dump", "--format=directory", "--file", db_dir, "--", db_name],
+                cwd=tmp,
+            )
             members.append("zulip-backup/database")
 
             if settings.LOCAL_UPLOADS_DIR is not None and os.path.exists(