add VCS information as a label in the image manifest

[jonboulle]

goaci should record some useful information into the image manifest (as labels and/or annotations).

I'd suggest the first useful bit of information would be a label referencing the git sha (or hg sha, ...) at which the image was built.

[krnowak]

I have implemented getting some information from all VCSes go itself supports, that is - git, hg, svn and bzr. Unfortunately I only tested git only.

My implementation simply creates a label with name like "git", "hg", "svn" or "bzr" and value with whatever "git rev-parse HEAD", "hg id -i", "svnversion" or "bzr revno" returns.