dch: make sure branch is always defined

This fixes a problem when running git-dch form a detached HEAD
Add --ignore-branch to gbp-pull

This allows us to fetch upstream and pristine-tar even when in detached
head state. This can be usefull when e.g. building in jenkins which uses
a detached head by default.
GitRepository Use _git_inout in get_branch

to suppress the confusiong "fatal: ref HEAD is not a symbolic ref". We
alrady put out a more descriptive error message. This makes using
--ignore-branch look far less dangerous.
gbp-pull: Fix typo in help output
manpages: Document gbp-pull's --ignore-branch option