Add support for git volumes.

I'm fine with revision for now - we went with revision for the top level name (generic) and commit for the nested git specific struct.  


> On Nov 3, 2014, at 10:47 AM, Brendan Burns <notifications@github.com> wrote:
> 
> In pkg/api/types.go:
> 
> > @@ -187,6 +189,15 @@ type GCEPersistentDisk struct {
> >  	ReadOnly bool `yaml:"readOnly,omitempty" json:"readOnly,omitempty"`
> >  }
> >  
> > +// GitRepo represents a volume that is pulled from git when the pod is created.
> > +type GitRepo struct {
> > +	// Repository URL
> > +	Repository string `yaml:"repository" json:"repository"`
> > +	// Commit hash, this is optional
> > +	Revision string `yaml:"revision" json:"revision"`
> No strong preferences. "commit" is an active word, that feels like the act of submitting, rather than "revision" which is more descriptive and feels like the state of the repository. Also "revision" feels more generally source control like, where-as commit is pretty git specific. I have no problems merging a PR in v1beta3 to make this "Commit" instead. (since it is a breaking api change) Anyone else have thoughts?
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