Supply update result value to the commit message template

[squaremo]

The effect of this PR is to give the commit message template some data to work with. Specifically: a data structure (definition given below) which holds the images updated for each object, for each file; and the name of the image automation object that created the commit.

This is the update result:

type Result struct {
	Files map[string]FileResult
}

type FileResult struct {
	Objects map[yaml.ResourceIdentifier][]name.Reference
}

.. where the map keys are file paths and {APIVersion, Kind, Name, Namespace}, and name.Reference is an image name.

Here's a template that uses the result structure to print out what was updated, in different ways:

template := `Commit summary

Image update automation object: {{ .AutomationObject }}

Files:
{{ range $filename, $_ := .Updated.Files -}}
- {{ $filename }}
{{ end -}}

Objects:
{{ range $resource, $_ := .Updated.Objects -}}
- {{ $resource.Kind }} {{ $resource.Name }}
{{ end -}}

Images:
{{ range .Updated.Images -}}
- {{.}}
{{ end -}}
`

(There are methods on the Result type to make enumerating the objects and image names possible without descending into the structure.)

EDIT While writing the guide I made the template data more convenient to use (see the commit):

• Write a reference for the template data
• Give the name of the policy for each update (A: yes it's useful)

EDIT Not sure whether these are worth it:

• [?] Wrap ResourceIdentifier in something that has convenient string-returning methods

[stefanprodan]

After reading the API docs I don't get how to use this feature, is this intended to place a go template inside messageTemplate? I think the api doc needs a YAML example.

[stefanprodan]

LGTM

Thanks @squaremo 🥇