debian/gbp.conf: Set multimaint-merge = True

Gbp-Dch: ignore

debian/gbp.conf

@@ -4,4 +4,5 @@ postbuild = ./prepare-release post-build
 debian-branch = debian/experimental
 debian-tag = %(version)s
 export-dir = ../build-area
-sign-tags = True
+sign-tags = True
+multimaint-merge = True