Implemented a function for developers to specify the --depth option of the git clone command

[yosssi]

Implemented a function for developers to specify the --depth option of the git clone command. refs #55

[scottferg]

LGTM

[bradrydzewski]

I just wrote a bunch of comments and forgot to hit submit. d'oh!

this is good stuff. I'd just like to propose some minor changes.

let's remove the SCM struct and just add the Git struct directly to script.Build. We have not plans to support non-git repositories at this time.

type Build struct {
   ...
   Git *Git `yaml:"git,omitempty"`
}

In the Git struct can we strongly type Depth as an int? The goyaml parser will unmarshal integer fields:

type Git struct {
   Depth int `yaml:"depth"`
}

NOTE: you may need to remove the omit empty, so that it gets populated with a 0, but I'm not sure.

Can we pass the Depth to the Repo as a field value? This is more for consistency, since this how we pass other values used in the Git command (sha, branch, etc):

type Repo struct {
   ...
   Depth int

so the method signature would look like this:

func (r *Repo) Commands() []string {

I appreciate the PR and especially the unit tests!

[yosssi]

Thanks for your reviewing.
I close this PR and I will create new one to merge the commits.

[yosssi]

See #62.