Use Model().Branches[0] instead of refsHelper.GetCheckedOutRef() in MergeAndRebaseHelper

It's the same, really, except that GetCheckedOutRef() does a check if any
branches exist and returns nil if not. Since we are accessing the returned
branch unconditionally without checking for nil, it seems this check is not
needed here. (The functions we are touching here are called from handlers that
are guarded with itemSelected or singleItemSelected, so we know that at least
one branch exists.)

The goal is to get rid of the dependency to refsHelper.

Author: stefanhaller

pkg/gui/controllers/helpers/merge_and_rebase_helper.go

@@ -270,8 +270,8 @@ func (self *MergeAndRebaseHelper) PromptToContinueRebase() error {
 }
 
 func (self *MergeAndRebaseHelper) RebaseOntoRef(ref string) error {
-	checkedOutBranch := self.refsHelper.GetCheckedOutRef()
-	checkedOutBranchName := self.refsHelper.GetCheckedOutRef().Name
+	checkedOutBranch := self.c.Model().Branches[0]
+	checkedOutBranchName := checkedOutBranch.Name
 	var disabledReason, baseBranchDisabledReason *types.DisabledReason
 	if checkedOutBranchName == ref {
 		disabledReason = &types.DisabledReason{Text: self.c.Tr.CantRebaseOntoSelf}
@@ -383,7 +383,7 @@ func (self *MergeAndRebaseHelper) MergeRefIntoCheckedOutBranch(refName string) e
 	if self.c.Git().Branch.IsHeadDetached() {
 		return errors.New("Cannot merge branch in detached head state. You might have checked out a commit directly or a remote branch, in which case you should checkout the local branch you want to be on")
 	}
-	checkedOutBranchName := self.refsHelper.GetCheckedOutRef().Name
+	checkedOutBranchName := self.c.Model().Branches[0].Name
 	if checkedOutBranchName == refName {
 		return errors.New(self.c.Tr.CantMergeBranchIntoItself)
 	}