Fix mode line inconsistency between normal and insert modes

In normal mode, the mode line contains "1 sel" or "n sels (k)" when n > 1,
whereas in insert mode, it contains "n sels (k)" even for n == 1. Change
the contents in insert mode to match normal mode.

src/input_handler.cc

@@ -1443,7 +1443,8 @@ public:
         auto main_index = context().selections().main_index();
         return {AtomList{ { "insert", context().faces()["StatusLineMode"] },
                           { " ", context().faces()["StatusLine"] },
-                          { format( "{} sels ({})", num_sel, main_index + 1),
+                          { num_sel == 1 ? format("{} sel", num_sel)
+                              : format("{} sels ({})", num_sel, main_index + 1),
                             context().faces()["StatusLineInfo"] } }};
     }