Extract a method CommitFilesContext.ReInit

Right now it doesn't do very much yet, but it's still worth it even in this
state, I'd say. The function is going to become a bit longer in the next commit,
though.

pkg/gui/context/commit_files_context.go

@@ -78,3 +78,9 @@ func (self *CommitFilesContext) GetDiffTerminals() []string {
 func (self *CommitFilesContext) ModelSearchResults(searchStr string, caseSensitive bool) []gocui.SearchPosition {
 	return nil
 }
+
+func (self *CommitFilesContext) ReInit(ref types.Ref) {
+	self.SetRef(ref)
+	self.SetTitleRef(ref.Description())
+	self.GetView().Title = self.Title()
+}