Git.Repository.__git_inout: properly set stdin

We should set stdin to subprocess.PIPE when piping input according
to the subprocess docs.
Git.Repository.__git_inout: Close all other file descriptors

No need to leak fds to the child.
.gitignore docbook xref

Git-Dch: Ignore
gbp.git.Repository.get_branch(): use _git_command

instead of the deprecated _git_inout and clarify the return codes and
exceptions raised.
gbp.git.GitRepository.has_branch(): use _git_command

instead of the deprecated _git_getoutput
Test succesful import too

and make sure we don't spew to stderr during this test
Make sure we keep our test coverage
Switch to debhelper level 9

which is available in Wheezy
Update Vcs-Browser URL
Add symlinks to the manpages for the deprecated commands

Closes: #714489

and make lintian happy
Fix broken xrefs