[release] prevent overwriting ${CHANGELOG}.orig with truncated file

to avoid deleting most of CHANGELOG.md by accident when the release.sh
script gets interrupted halfway through, as happened during the v1.28.3
release in commit 7e8ca377fc

gallery_dl/version.py

@@ -6,5 +6,5 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
-__version__ = "1.28.4"
+__version__ = "1.28.5-dev"
 __variant__ = None