utimecmp: fine-grained src to nearby coarse-grained dest

utimecmp: fine-grained src to nearby coarse-grained dest

* lib/utimecmp.c (utimecmp): When UTIMECMP_TRUNCATE_SOURCE is set,
and the source is on a file system with higher-resolution time
stamps, than the destination, and _PC_TIMESTAMP_RESOLUTION does
not work, and the time stamps are close together, the algorithm to
determine the exact resolution from the read-back mtime was buggy:
it had a "!=" where it should have had an "==".  This bug has been
in the code ever since it was introduced to gnulib.
Problem reported by Dan Jacobson in
<http://debbugs.gnu.org/cgi/bugreport.cgi?bug=7529>.