readme-release: improve safety of release prep instructions.

readme-release: improve safety of release prep instructions.

* README-release: Don't git pull all branches when only master
is needed for the release process.
Run make maintainer-clean before changing trees and merging.
Don't try to run ./configure right after git pull in case files
that influence the bootstrap process have changed, move the
./configure step to after running ./bootstrap.
Don't bootstrap "one last time"... it's the first time!