Extract git flow init from release script (so that it can be run standalone) and do not use `-t` which does not seem to be standard.

tools/gitflow/gitflow-init.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+#
+# Copyright (c) 2023 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+git flow init -d
+git config gitflow.prefix.versiontag v

tools/release/release.sh

@@ -86,8 +86,7 @@ then
     printf "Git flow is initialized\n"
 else
     printf "Git flow is not initialized. Initializing...\n"
-    # All default value, just set 'v' for tag prefix
-    git flow init -d -t 'v'
+    ./tools/gitflow/gitflow-init.sh
 fi
 
 printf "OK\n"