generate-git-snapshot: use --git-ignore-branch as default build optio for gbp [Closes #47]

As reported in https://github.com/mika/jenkins-debian-glue/issues/47

Compare:

| % git-buildpackage -nc --git-force-create --git-ignore-new -S -us -uc --git-verbose --git-builder=/bin/true --git-cleaner=/bin/true
| gbp:debug: ['git', 'rev-parse', '--show-cdup']
| gbp:debug: ['git', 'rev-parse', '--is-bare-repository']
| gbp:debug: /bin/true [] []
| gbp:debug: ['git', 'symbolic-ref', 'HEAD']
| gbp:error: Currently not on a branch

with:

| % git-buildpackage -nc --git-force-create --git-ignore-new -S -us -uc --git-verbose --git-builder=/bin/true --git-cleaner=/bin/true --git-ignore-branch
| gbp:debug: ['git', 'rev-parse', '--show-cdup']
| gbp:debug: ['git', 'rev-parse', '--is-bare-repository']
| gbp:debug: /bin/true [] []
| gbp:debug: ['git', 'symbolic-ref', 'HEAD']
| gbp:debug: ['git', 'show-ref', 'refs/heads/pristine-tar']
| gbp:debug: ['git', 'ls-tree', 'upstream/1.7.4']
| gbp:error: upstream/1.7.4 is not a valid treeish

using a recent git-buildpackage version.

Verified against git-buildpackage 0.6.0~git20130506.
As the option is already present (at least) in gbp 0.5.10
this should be a safe way how to prevent problems with
recent versions of gbp.