Far too many modifications for a single commit, need to restart.

* bzrlib/tests/workingtree_implementations/test_parents.py:
(TestSetParents.test_unicode_symlink): Add more checks for
involved trees.

* bzrlib/tests/tree_implementations/test_get_symlink_target.py:
(TestGetSymlinkTarget.test_get_unicode_symlink_target): Bah, no
need to test unicode file-ids here.

* bzrlib/tests/test_transform.py:
(TestBuildTree.test_case_insensitive_build_tree_inventory): Fix
the test condition for good.

* bzrlib/tests/test_shelf.py:
Fix imports.
(TestPrepareShelf._test_shelve_symlink_target_change): Factored
out.
(TestPrepareShelf.test_shelve_symlink_target_change)
(TestPrepareShelf.test_shelve_unicode_symlink_target_change): Also
test unicode symlinks.

* bzrlib/tests/test_bundle.py:
(BundleTester._test_symlink_bundle): Factored out.
(BundleTester.test_symlink_bundle,
BundleTester.test_unicode_symlink_bundle): Also test unicode
symlinks.

* bzrlib/tests/test__dirstate_helpers.py:
Fix some imports.
(load_tests): Start parametrizing against DirReaders.

* bzrlib/merge_directive.py:
(MergeDirectiveFormatRegistry): Add Aaron explanation for the
double registration.

* bzrlib/hashcache.py:
(HashCache.get_sha1): Instrumented for pronto.

* bzrlib/revisiontree.py:
(RevisionTree.get_symlink_target): Add comment.

* bzrlib/osutils.py:
(_walkdirs_utf8, UnicodeDirReader.read_dir): Intrumented for pronto.

* bzrlib/dirstate.py:
(DirState._read_link): Intrumented for pronto.

* bzrlib/tests/per_repository/test_commit_builder.py:
(TestCommitBuilder._add_commit_change_check_changed): Add a
file_id optional parameter.
(TestCommitBuilder._test_last_mod_rev_after_content_link_changes_ric):
Factored out.
(TestCommitBuilder.test_last_modified_rev_after_content_link_changes_ric,
TestCommitBuilder.test_last_modified_rev_after_content_unicode_link_changes_ric):
Also test unicode symlinks.

* bzrlib/tests/branch_implementations/test_sprout.py:
(TestSprout.test_sprout_with_unicode_symlink): Use an unicode name
for the link too. Add more checks for involved trees.