create-release-branches.bash: refactor

- extract listK3sRepoBranches
- fix error

 line 10: [: ==: unary operator expected

create-release-branches.bash

@@ -7,13 +7,18 @@ set -e # exit on non-zero status
 # When PUSH_OPTIONS=true, it also recreates all existing release branches from master branch
 
 PUSH_OPTIONS=""
-if [ $FORCE_PUSH = "true" ] ;then
+if [ "$FORCE_PUSH" == "true" ] ;then
   PUSH_OPTIONS="$PUSH_OPTIONS --force"
 fi
 current_version=$(yq -r '.directories[0].contents[] | select (.path=="k3s-io/k3s") | .githubRelease.tag ' ./vendir.yml)
 current_version=${current_version#v}
 MIN_VERSION=$(echo "$current_version"|cut -d'.' -f1-2)
-for ref in $(git ls-remote -h https://github.com/k3s-io/k3s "release-*" | sed 's/refs\/heads\///' | awk '{print $2}') ; do
+
+function listK3sRepoBranches() {
+  git ls-remote -h https://github.com/k3s-io/k3s "release-*" | sed 's/refs\/heads\///' | awk '{print $2}'
+}
+
+for ref in $(listK3sRepoBranches) ; do
   v=${ref#release-}
   BRANCH_NAME="${ref}"
   echo "Extracted values - v: $v - BRANCH_NAME: $BRANCH_NAME"