dirstate: ignore stat data for files that were updated too recently
This should fix the race where
hg commit foo
<change foo without changing its size>
happens in the same second and status is fooled into thinking foo
is clean.
A configuration item is used to determine the timeout, since different
filesystems may have different requirements (I think VFAT needs 3s,
while most Unix filesystems are fine with 1s).
adding bar
adding foo
% state dump
a 0 -1 baz
n 0 -1 foo
r 0 0 bar
% status
A baz
R bar
C foo
% state dump
n 666 -1 bar
n 666 -1 foo
% status
! bar
? baz
C foo