Fix timestamp bug when write-region appends nothing.

Fix timestamp bug when write-region appends nothing.

* fileio.c (Fwrite_region): When neither O_EXCL nor O_TRUNC is used,
the file's time stamp doesn't change if Emacs happens to write nothing
to the file, and on a buggy file system this could cause Emacs to
incorrectly infer that the file system doesn't have the bug.
Avoid this problem by inhibiting the inference in this case.