Fix prevention of creating branch from wrong source branch

Author: Tobias Meinhardt

git/native_adapter.go

@@ -112,13 +112,17 @@ func (a nativeGitAdapter) Push(setUpstream bool) error {
 
 // Create a new branch
 func (a nativeGitAdapter) Create(b glow.Branch) error {
-	if !b.CreationIsAllowedFrom(b.BranchName()) {
+	sourceBranch, err := a.CurrentBranch()
+	if err != nil {
+		return err
+	}
+	if !b.CreationIsAllowedFrom(sourceBranch.BranchName()) {
 		return errors.New("creation not allowed from this branch")
 	}
 	cmd := a.exec.Command(fmt.Sprintf("git branch %s", b.ShortBranchName()))
 	var stderr bytes.Buffer
 	cmd.Stderr = &stderr
-	err := cmd.Run()
+	err = cmd.Run()
 	return errors.Wrap(err, stderr.String())
 }