Fix longstanding bug (chdir to nonexistent dirs).

In test, set up the post-commit hook for more realism (and bugs!).
To make wrappers work in test, set PERL5LIB, and allow the wrappee's
path to be overridden. Meta-test that post-commit is really hooked
up by verifying that content is getting generated in destdir.

About the longstanding bug, which as far as I know was harmless:
CVS can't operate outside a srcdir, so we're always setting $CWD.
"local $CWD" restores the previous value when we go out of scope.
Usually that's correct. But if we're removing the last file from a
directory, the post-commit hook will exec in a working directory
that's about to not exist (CVS will prune it).

The fix: chdir() manually in cvs_runcvs(), so we can selectively
not chdir() back.