Store fake keystrokes when selecting an insert completion candidate

Instead of storing the real 'select next completion' keystroke,
generate fake backspace/delete/char keystrokes so that replay
of the insert will insert the exact same text.

Fixes #135

src/input_handler.cc

@@ -879,12 +879,14 @@ public:
             insert('\t');
         else if (key == ctrl('n'))
         {
-            m_completer.select(1);
+            last_insert().second.pop_back();
+            m_completer.select(1, last_insert().second);
             update_completions = false;
         }
         else if (key == ctrl('p'))
         {
-            m_completer.select(-1);
+            last_insert().second.pop_back();
+            m_completer.select(-1, last_insert().second);
             update_completions = false;
         }
         else if (key == ctrl('x'))

src/insert_completer.cc

@@ -245,7 +245,7 @@ InsertCompleter::~InsertCompleter()
     m_options.unregister_watcher(*this);
 }
 
-void InsertCompleter::select(int offset)
+void InsertCompleter::select(int offset, Vector<Key>& keystrokes)
 {
     if (not setup_ifn())
         return;
@@ -283,6 +283,13 @@ void InsertCompleter::select(int offset)
             m_context.ui().info_show(candidate.first, candidate.second, CharCoord{},
                                      get_face("Information"), InfoStyle::MenuDoc);
     }
+
+    for (auto i = 0_byte; i < prefix_len; ++i)
+        keystrokes.push_back(Key::Backspace);
+    for (auto i = 0_byte; i < suffix_len; ++i)
+        keystrokes.push_back(Key::Delete);
+    for (auto& c : candidate.first)
+        keystrokes.push_back(c);
 }
 
 void InsertCompleter::update()

src/insert_completer.hh

@@ -60,7 +60,7 @@ public:
     InsertCompleter& operator=(const InsertCompleter&) = delete;
     ~InsertCompleter();
 
-    void select(int offset);
+    void select(int offset, Vector<Key>& keystrokes);
     void update();
     void reset();