provide a default value of the branch

Author: LinuxSuRen

README.md

@@ -18,7 +18,7 @@ GitHub Action for backup your git repository.
 
 - `GIT_DEPLOY_KEY`: **Required**, your deploy key (ssh private key) which has **Write access**
 - `TARGET_GIT`: **required**, your backup git repository (please use ssh or git protocol)
-- `BRANCH`: **required**, the branch you want to backup
+- `BRANCH`: **optional**, the branch you want to backup (default value is `master`)
 
 ```
 name: Backup Git Repository

entrypoint.sh

@@ -12,13 +12,13 @@ git config --global --add safe.directory /github/workspace
 
 git version
 git fetch --unshallow origin
-git checkout ${BRANCH}
+git checkout ${BRANCH:-master}
 git status
 git remote add backup ${TARGET_GIT}
 git remote -vv
 git config --global --add safe.directory /github/workspace
 git config --global user.name "${GITHUB_ACTOR}"
 git config --global user.email "${GITHUB_ACTOR}@users.noreply.github.com"
 echo 'ready to push'
-git push backup ${BRANCH} -f
+git push backup ${BRANCH:-master} -f
 git remote remove backup