use reflog undo history pointer

pkg/gui/gui.go

@@ -214,6 +214,21 @@ type guiState struct {
 	PrevMainWidth        int
 	PrevMainHeight       int
 	OldInformation       string
+
+	Undo UndoState
+}
+
+// we facilitate 'undo' actions via parsing the reflog and doing the reverse
+// of the most recent entry. In order to to multiple undo's in a row we need to
+// keep track of where we are in the reflog. We do that via a key which is the
+// concatenation of the reflog's timestamp and message.
+// We also store the index of that reflog entry so that if we end up with a
+// different entry at that index we know the user must have done something
+// themselves (e.g. checked out a branch) to cause new reflog entries to be created
+// meaning we can reset the undo state.
+type UndoState struct {
+	ReflogKey string
+	ReflogIdx int
 }
 
 // for now the split view will always be on