files: don't bother counting rows when opening a fresh buffer

Counting the added number of rows is only relevant when inserting
a file into the current buffer.  So don't waste time counting when
it's not needed.

This fixes https://savannah.gnu.org/bugs/?51479.

src/files.c

@@ -926,7 +926,7 @@ void read_file(FILE *f, int fd, const char *filename, bool undoable,
 			(unsigned long)num_lines), (unsigned long)num_lines);
 
     /* If we inserted less than a screenful, don't center the cursor. */
-    if (less_than_a_screenful(was_lineno, was_leftedge))
+    if (undoable && less_than_a_screenful(was_lineno, was_leftedge))
 	focusing = FALSE;
 
 #ifndef NANO_TINY