release.pl: properly use "git describe"

We weren't chomping the newline, and we didn't want `--tags` since this
includes non-annotated tags like `last-svn-trunk-commit`.

No reviewer.
Revert "check accumulate datatype-op compatibility"

This reverts commit ba2e444468ca13d559874e2a27229503efa693c2.

This was causing `accfence1` (and other) failures.  Reverting for now,
since the apparently easy fix is not quite as simple as it first
appears.