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: a101211279 <1012112796@qq.com>

Co-authored-by: a101211279 <1012112796@qq.com>

models/action.go

@@ -13,6 +13,7 @@ import (
 	"time"
 
 	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 	"code.gitea.io/gitea/modules/timeutil"
@@ -238,7 +239,7 @@ func (a *Action) getCommentLink(e Engine) string {
 
 // GetBranch returns the action's repository branch.
 func (a *Action) GetBranch() string {
-	return a.RefName
+	return strings.TrimPrefix(a.RefName, git.BranchPrefix)
 }
 
 // GetContent returns the action's content.