Merge overlapping selections before erasing

Overlapping selections on erase confuse the ForwardChangeTracker
and will confuse the user,

Fixes #298

src/selection.cc

@@ -508,6 +508,10 @@ void SelectionList::insert(ConstArrayView<String> strings, InsertMode mode,
 void SelectionList::erase()
 {
     update();
+
+    m_selections.erase(merge_overlapping(begin(), end(), m_main, overlaps),
+                       end());
+
     ForwardChangesTracker changes_tracker;
     for (auto& sel : m_selections)
     {
@@ -529,6 +533,7 @@ void SelectionList::erase()
         if (sel.cursor() > back_coord)
             sel.cursor() = back_coord;
     }
+
     m_buffer->check_invariant();
 }
 

test/compose/del-empty-line/cmd

@@ -0,0 +1 @@
+CCdd

test/compose/del-empty-line/in

@@ -0,0 +1,3 @@
+asdf
+
+asdf

test/compose/del-empty-line/out

@@ -0,0 +1,2 @@
+df
+df