titlebar: don't reshow version when dropping back to a single buffer

When multiple files were open and [x/n] was being shown in the title
bar, don't show nano's name and version number when just one buffer
remains open, but show [1/1] instead.  It is less surprising.

src/files.c

@@ -78,6 +78,7 @@ void make_new_buffer(void)
 
 	/* There is more than one file open: show "Close" in help lines. */
 	exitfunc->desc = close_tag;
+	more_than_one = TRUE;
     }
 
     /* Make the new buffer the current one, and start initializing it. */