Trim the branch prefix from action.GetBranch (#13981)

Trim the branch prefix from action.GetBranch (#13981)

* Trim the branch prefix from action.GetBranch

 #13882 has revealed that the refname of an action is actually only a
refname pattern and necessarily a branch. For examplem pushing to
refs/heads/master will result in action with refname refs/heads/master
but pushing to master will result in a refname master.

The simplest solution to providing a fix here is to trim the prefix
therefore this PR proposes this.

Signed-off-by: Andrew Thornton <art27@cantab.net>
* Update models/action.go

Co-authored-by: a1012112796 <1012112796@qq.com>
Co-authored-by: a1012112796 <1012112796@qq.com>