util: disable floating point stat times (issue4836)

Alternate fix for this issue which avoids putting extra function calls
and exception handling in the fast path.

For almost all purposes, integer timestamps are preferable to
Mercurial. It stores integer timestamps in the dirstate and would thus
like to avoid doing any float/int comparisons or conversions. We will
continue to have to deal with 1-second granularity on filesystems for
quite some time, so this won't significantly hinder our capabilities.

This has some impact on our file cache validation code in that it
lowers timestamp resolution. But as we still have to deal with
low-resolution filesystems, we're not relying on this anyway.

An alternate approach is to use stat[ST_MTIME], which is guaranteed to
be an integer. But since this support isn't already in our extension,
we can't depend on it being available without adding a hard Python->C
API dependency that's painful for people like yours truly who have
bisect regularly and people without compilers.

mercurial/util.py

@@ -88,6 +88,11 b' username = platform.username'
 
 _notset = object()
 
+# disable Python's problematic floating point timestamps (issue4836)
+# (Python hypocritically says you shouldn't change this behavior in
+# libraries, and sure enough Mercurial is not a library.)
+os.stat_float_times(False)
+
 def safehasattr(thing, attr):
     return getattr(thing, attr, _notset) is not _notset