Fix the CurrentBranchName function

The function would return "head/branchname" when there was either a tag or a
remote with the same name.

While fixing this we slightly change the semantic of the function (and of
determineCheckedOutBranchName, which calls it): for a detached head it now
returns an empty string rather than the commit hash. I actually think this is
better.

pkg/gui/controllers/helpers/refresh_helper.go

@@ -308,7 +308,7 @@ func (self *RefreshHelper) determineCheckedOutBranchName() string {
 	// In all other cases, get the branch name by asking git what branch is
 	// checked out. Note that if we're on a detached head (for reasons other
 	// than rebasing or bisecting, i.e. it was explicitly checked out), then
-	// this will return its hash.
+	// this will return an empty string.
 	if branchName, err := self.c.Git().Branch.CurrentBranchName(); err == nil {
 		return branchName
 	}