edit.c: CTRL-SPC: Insert previously-inserted text. #6090

Default Vim behavior of i_CTRL-<Space> is to insert the last-inserted
text and exit insert mode. :help i_CTRL-@

Before this commit that did not happen because insert_handle_key()
checks for NUL instead of checking for ' ' with a CTRL `mod_mask`.

I'm leaving the check for NUL despite the fact that at the moment that
key is never seen when using the terminal UI (not for C-Space, nor C-@).
This is because I assume it's still allowed for other front-ends to pass
NUL, but at the moment the terminal UI isn't.

src/nvim/edit.c

@@ -844,6 +844,11 @@ static int insert_handle_key(InsertState *s)
     return 0;  // exit insert mode
 
 
+  case ' ':
+    if (mod_mask != 4) {
+      goto normalchar;
+    }
+  // FALLTHROUGH
   case K_ZERO:        // Insert the previously inserted text.
   case NUL:
   case Ctrl_A: