build.xml improvements

Modify -pre-bump-check so that it verifies that there is no
existing git tag for the new version.
Eliminate non-working text replacements

All \r and \n codes have been replaced with <br />, so the patterns in
these replacements don't match anything.

This problem has existed for some time -- since commits 1ea27d7 and
e12dd32.

No attempt is made here to reimplement the replacements because users are
now used to the current behavior without much apparent complaint, and such
replacements are never perfect and can occasionally fail to work as
desired without additional tweaking for special cases.