dirstate: fix debug.dirstate.delaywrite to use the new "now" after sleeping

It seems like the a regression has sneaked into debug.dirstate.delaywrite in
6c6b48aca328. It would sleep until no files were modified "now" any more, but
when writing the dirstate it would use the old "now" and still mark files as
'unset' instead of recording the timestamp that would make the file show up as
clean instead of unknown.

Instead of getting a new "now" from the file system, we trust the computed end
time as the new "now" and thus cause the actual modification time to be
writiten to the dirstate.

debug.dirstate.delaywrite is undocumented and only used in
test-largefiles-update.t . All tests seems to work fine for me without
debug.dirstate.delaywrite . Perhaps because it not really worked as intended
without the fix in this patch, and code and tests thus have evolved to do fine
without it? It could thus perhaps make sense to drop usage of this setting in
the tests. That could speed the test up a bit.

This functionality (or something very similar) can however apparently be very
convenient in setups where checking dirty-ness is expensive - such as when
using large files and have slow file filesystems or are CPU constrained. Now it
works and we can try it. (But ideally, for the largefile use case, it should
probably only delay lfdirstate writes - not ordinary dirstate.)

mercurial/dirstate.py

@@ -757,6 +757,7 b' class dirstate(object):'
                     start = int(clock) - (int(clock) % delaywrite)
                     end = start + delaywrite
                     time.sleep(end - clock)
+                    now = end # trust our estimate that the end is near now
                     break
 
         st.write(parsers.pack_dirstate(self._map, self._copymap, self._pl, now))