tests/test-dirstate-race2.t

cext: add Python 3.10 support

* Replace "Py_TYPE(obj) = type;" with "Py_SET_TYPE(obj, type);"
* Add pythoncapi_compat.h header file to get Py_SET_TYPE() on Python
  2.7-3.8. Header file added to mercurial/ and
  contrib/python-zstandard/zstd/common/.

In Python 3.10, Py_TYPE(obj) must not longer be used as an l-value.

pythoncapi_compat.h comes from:
https://github.com/pythoncapi/pythoncapi_compat

Differential Revision: https://phab.mercurial-scm.org/D9825

Checking the size/permissions/file-type of files stored in the
dirstate after an update where the files are changed concurrently
outside of hg's control.

  $ hg init repo
  $ cd repo
  $ echo a > a
  $ hg commit -qAm _
  $ echo aa > a
  $ hg commit -m _

  $ hg debugdirstate --no-dates
  n 644          3 (set  |unset)               a (re)

  $ cat >> $TESTTMP/dirstaterace.py << EOF
  > from mercurial import (
  >     extensions,
  >     merge,
  > )
  > def extsetup(ui):
  >     extensions.wrapfunction(merge, 'applyupdates', wrap)
  > def wrap(orig, *args, **kwargs):
  >     res = orig(*args, **kwargs)
  >     with open("a", "w"):
  >         pass # just truncate the file
  >     return res
  > EOF

Do an update where file 'a' is changed between hg writing it to disk
and hg writing the dirstate. The dirstate is correct nonetheless, and
so hg status correctly shows a as clean.

  $ hg up -r 0 --config extensions.race=$TESTTMP/dirstaterace.py
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg debugdirstate --no-dates
  n 644          2 (set  |unset)               a (re)
  $ echo a > a; hg status; hg diff