BF: replace show-ref with rev-parse --verify for determining current treeish location

--HG--
extra : rebase_source : 5e91ffb0a20724dc92084bb68288cb6000814dca

generate safer cleanup code that also works in the case something goes badly wrong early at module init time

--HG--
extra : rebase_source : 015495e6aee6c59b11ee9169d2ad597ff54bfd0c

Conflicts:
	CHANGES.rst
	Cython/__init__.py

make AnalyseDeclarationsTransform inherit from EnvTransform to fix inconsistencies in scope tracking