Only commit undo groups when buffer was modified

This fixes an interaction with kak-lsp that would trigger undo group
commit when setting a buffer option value.

src/context.cc

@@ -211,7 +211,11 @@ Vector<String> Context::selections_content() const
 void Context::begin_edition()
 {
     if (m_edition_level >= 0)
+    {
+        if (m_edition_level == 0)
+            m_edition_timestamp = buffer().timestamp();
         ++m_edition_level;
+    }
 }
 
 void Context::end_edition()
@@ -220,7 +224,8 @@ void Context::end_edition()
         return;
 
     kak_assert(m_edition_level != 0);
-    if (m_edition_level == 1)
+    if (m_edition_level == 1 and
+        buffer().timestamp() != m_edition_timestamp)
         buffer().commit_undo_group();
 
     --m_edition_level;

src/context.hh

@@ -134,6 +134,7 @@ private:
     void begin_edition();
     void end_edition();
     int m_edition_level = 0;
+    size_t m_edition_timestamp = 0;
 
     friend struct ScopedEdition;
 

test/regression/0-spurious-undo-group-on-external/cmd

@@ -0,0 +1 @@
+Abarbaz<esc>u

test/regression/0-spurious-undo-group-on-external/in

@@ -0,0 +1 @@
+foo

test/regression/0-spurious-undo-group-on-external/out

@@ -0,0 +1 @@
+foo

test/regression/0-spurious-undo-group-on-external/rc

@@ -0,0 +1,2 @@
+decl int my_option
+hook global InsertChar r %{ eval -buffer %val{buffile} %{ eval "set buffer my_option 1" } }