grid: reflow: require col count > 0 when skipping line truncation

When reflowing the grid, we truncate lines with a hard linebreak after
the last non-empty cell. This way we don’t reflow trailing empty cells
to a new line when resizing the window to a smaller size.

However, “logical” lines (i.e. those without a hard linebreak)
are *not* truncated. This is to ensure we don’t trim empty cells in
the middle of a logical line spanning multiple physical lines.

Since newly allocated rows are initialized with linebreak=false, we
need to ensure _those_ are still truncated - otherwise all that empty
space under the current prompt will be reflowed.

Note that this is a temporary workaround. The correct solution, I
think, is to track whether a line has been printed to or not, and
simply ignore (not reflow) lines that haven’t yet been touched.

grid.c

@@ -740,7 +740,7 @@ grid_resize_and_reflow(
             }
         }
 
-        if (!old_row->linebreak /*&& col_count > 0*/) {
+        if (!old_row->linebreak && col_count > 0) {
             /* Don’t truncate logical lines */
             col_count = old_cols;
         }