Changes file headers

git-ubuntu understands the following custom changes file headers:

Vcs-Git

Points to the git repository where the rich history can be found.

Vcs-Git-Ref

The ref which when fetched contains the rich history.

Vcs-Git-Commit

The commit hash of your rich history. This must match your upload.

When git-ubuntu imports your upload, it will look in the location specified by these headers for the rich history. If present and if they match your upload, then it will use your commit instead of synthesizing its own.

For now, only Launchpad git URLs are accepted to avoid the risk from a malicious git repository host. git-ubuntu prepare-upload will check that the URL will be acceptable.