streamclone: disable the volatile file open handle optimization on Windows

Leaving files open caused new failures like this, since a47f09da8bd1:

    diff --git a/tests/test-persistent-nodemap-stream-clone.t b/tests/test-persistent-nodemap-stream-clone.t
    --- a/tests/test-persistent-nodemap-stream-clone.t
    +++ b/tests/test-persistent-nodemap-stream-clone.t
    @@ -115,7 +115,12 @@ Do a mix of clone and commit at the same
       $ (hg clone -U --stream ssh://user@dummy/test-repo stream-clone-race-1 --debug 2>> clone-output | grep -E '00(changelog|manifest)' >> clone-output; touch $HG_TEST_STREAM_WALKED_FILE_3) &
       $ $RUNTESTDIR/testlib/wait-on-file 10 $HG_TEST_STREAM_WALKED_FILE_1
       $ hg -R test-repo/ commit -m foo
    -  created new head
    +  transaction abort!
    +  failed to recover 00changelog.n ([WinError 32] The process cannot access the file because it is being used by another process: b'$STR_REPR_TESTTMP\\test-repo/.hg/store/00changelog.n' -> b'$STR_REPR_TESTTMP\\test-repo/.hg/store/00changelog.n-f418dcd6')
    +  rollback failed - please run hg recover
    +  (failure reason: [WinError 32] The process cannot access the file because it is being used by another process: b'$STR_REPR_TESTTMP\\test-repo/.hg/store/00changelog.n' -> b'$STR_REPR_TESTTMP\\test-repo/.hg/store/00changelog.n-f418dcd6')
    +  abort: The process cannot access the file because it is being used by another process: '$TESTTMP\test-repo\.hg\store\00changelog.n'
    +  [255]
       $ touch $HG_TEST_STREAM_WALKED_FILE_2
       $ $RUNTESTDIR/testlib/wait-on-file 10 $HG_TEST_STREAM_WALKED_FILE_3
       $ cat clone-output

Since the `VolatileManager` falls back to the old copy method when the open file
threshold is exceeded, this just drops the threshold so that only 1 file is
open.  The actual value used (2) is unexpected, and explained inline.  I'd like
to have a config option for this so that we can test both ways (in theory, it
could resort to copies on non-Windows systems too), but I don't see a `uimod.ui`
handy.

Alternately, I tried replacing the 3 `open()` calls in the `VolatileManager`
with `util.posixfile()`, but that simply hung the test on Windows for some
reason, I think on the same line that's indicated as failing above.  (There was
a `grep` command hanging around, as well as `hg  -R test-repo serve --stdio`.)

mercurial/streamclone.py

@@ -582,8 +582,10 class VolatileManager:
     """
 
     # arbitrarily picked as "it seemed fine" and much higher than the current
-    # usage.
-    MAX_OPEN = 100
+    # usage.  The Windows value of 2 is actually 1 file open at a time, due to
+    # the `flush_count = self.MAX_OPEN // 2` and `self.MAX_OPEN - 1` threshold
+    # for flushing to disk in __call__().
+    MAX_OPEN = 2 if pycompat.iswindows else 100
 
     def __init__(self):
         self._counter = 0