configury: make the release rules more robust.

* Makefile.am (GIT, GIT_REMOTE, LN_S): Set as macros that can be
overridden during testing.
Adjust all callers.
(tag-release, check-in-release): Move the push commands that
publish changes upstream from here...
(release): ...to here.
(check-in-release): Rather than rely on having a parallel checkout
in a particular sibling directory, just switch branches from here
temorarily.
(unpack-distcheck-release): Once in the relase branch, overwrite
the previous release with the tarball from distcheck.