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. Fixes: debbugs:13149
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