Support merging consecutive selections with <a-m>

Fixes #773

src/normal.cc

@@ -1590,6 +1590,12 @@ void ensure_forward(Context& context, NormalParams)
     context.selections().check_invariant();
 }
 
+void merge_consecutive(Context& context, NormalParams params)
+{
+    ensure_forward(context, params);
+    context.selections().merge_consecutive();
+}
+
 void force_redraw(Context& context, NormalParams)
 {
     if (context.has_client())
@@ -1663,6 +1669,7 @@ static NormalCmdDesc cmds[] =
     { ';', "reduce selections to their cursor", clear_selections },
     { alt(';'), "swap selections cursor and anchor", flip_selections },
     { alt(':'), "ensure selection cursor is after anchor", ensure_forward },
+    { alt('m'), "merge consecutive selections", merge_consecutive },
 
     { 'w', "select to next word start", repeated<&select<SelectMode::Replace, select_to_next_word<Word>>> },
     { 'e', "select to next word end", repeated<select<SelectMode::Replace, select_to_next_word_end<Word>>> },

src/selection.cc

@@ -445,6 +445,18 @@ void SelectionList::merge_overlapping()
                                                   m_main, overlaps), end());
 }
 
+void SelectionList::merge_consecutive()
+{
+    if (size() == 1)
+        return;
+
+    auto touches = [this](const Selection& lhs, const Selection& rhs) {
+        return m_buffer->char_next(lhs.max()) >= rhs.min();
+    };
+    m_selections.erase(Kakoune::merge_overlapping(begin(), end(),
+                                                  m_main, touches), end());
+}
+
 void SelectionList::sort_and_merge_overlapping()
 {
     sort();

src/selection.hh

@@ -126,6 +126,7 @@ struct SelectionList
 
     void sort();
     void merge_overlapping();
+    void merge_consecutive();
     void sort_and_merge_overlapping();
 
     Buffer& buffer() const { return *m_buffer; }