Fix invalid line joining logic with multiple selection per line

Fixes #4476

src/normal.cc

@@ -1104,6 +1104,7 @@ void join_lines_select_spaces(Context& context, NormalParams)
     if (selections.empty())
         return;
     context.selections_write_only() = std::move(selections);
+    context.selections().merge_consecutive();
     ScopedEdition edition(context);
     context.selections().replace({" "_str});
 }

test/regression/4476-invalid-line-join/cmd

@@ -0,0 +1 @@
+%<a-s><a-S><a-j>

test/regression/4476-invalid-line-join/in

@@ -0,0 +1,2 @@
+ab
+ac

test/regression/4476-invalid-line-join/out

@@ -0,0 +1 @@
+ab ac