tests/test-narrow-debugrebuilddirstate.t

setup: exclude the git extension from py2 builds

This can't be built on Windows with the py2 compiler, and while old versions can
be installed via pip on Linux, I can't get the tests to run (even with py3.8)
using pygit2 0.28.2.  Some manually run commands work, and others spew stack
traces that don't occur with the current 1.4.0 release using py3.

Differential Revision: https://phab.mercurial-scm.org/D9604

  $ . "$TESTDIR/narrow-library.sh"
  $ hg init master
  $ cd master
  $ echo treemanifest >> .hg/requires
  $ echo 'contents of file' > file
  $ mkdir foo
  $ echo 'contents of foo/bar' > foo/bar
  $ hg ci -Am 'some change'
  adding file
  adding foo/bar

  $ cd ..
  $ hg clone --narrow ssh://user@dummy/master copy --include=foo
  requesting all changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  new changesets * (glob)
  updating to branch default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd copy

  $ hg debugdirstate --no-dates
  n *         20 *               foo/bar (glob)
  $ mv .hg/dirstate .hg/old_dirstate
  $ dd bs=40 count=1 if=.hg/old_dirstate of=.hg/dirstate 2>/dev/null
  $ hg debugdirstate
  $ hg debugrebuilddirstate
  $ hg debugdirstate
  n *         * unset               foo/bar (glob)