git-debrebase: Split into its own package

git-debrebase: Split into its own package

* Provide and use Debian::Dgit::GDR which does the same for @INC as
  Debian::Dgit:Infra, only for GDR.  It's not *entirely* similar as
  its doc comment talks about ExitStatus too.

* In Makefile, split off lots of GDR_* variables and the
  two targets install-gdr and installdirs-gdr
  We take no care that only the required manpages are built for
  each install target; instead, they all go in MANPAGES and `all'.

* Add a control file stanza.  git-debrebase takes over the dependenc
  on libfile-fnmatch-perl.  We Recommend dgit and gbp.  (Even though
  if you're working with neither the Debian archive nor `3.0 (quilt)',
  you need neither.)

* In rules, add specpkg_install_gdr, to run the Makefile machinery.

* In the test dependencies, GDR now means to ask for git-debrebase and
  also git-buildpackage (for make-patches).  libfile-fnmatch-perl is
  handled via the dependencies of git-debrebase.deb.

* Add a .gitignore for the staging area in debian/

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>