use NULL instead of NA. include git revision and date stamp in build log. generate...

use NULL instead of NA. include git revision and date stamp in build log. generate changelog.

NA has length 1 and conveys perhaps-ok information. NULL was often more
appropriate.

a configure script edits R/zzz.R to add a new global 'git_revision'
prior to R building the package. This assumes that the current working
directory is in a git repository and will fail if it is not.

changelog generated according to Debian guidelines---latest builds to
the top. Included: the git revision of cran2deb, the time/date, and the
database version.

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/cran2deb@111 edb9625f-4e0d-4859-8d74-9fd3b1da38cb