Change behavior for the existing filename

Previously we always picked the last file that was openend as the file
name to save to. That seems counterintuitive when importing files or when
opening multiple files. Especially if Subsurface was executed without a
file on the command line and we are using the default file.

Now we only remember a file name if it was the first one to ever be
openend or if it was used in save-as.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

main.c

@@ -243,7 +243,7 @@ int main(int argc, char **argv)
 		parse_file(filename, &error);
 		/* don't report errors - this file may not exist, but make
 		   sure we remember this as the filename in use */
-		set_filename(filename);
+		set_filename(filename, FALSE);
 	}
 	report_dives(imported);