Abstract hash function usage (#28138)

Refactor Hash interfaces and centralize hash function. This will allow
easier introduction of different hash function later on.

This forms the "no-op" part of the SHA256 enablement patch.

services/webhook/slack.go

@@ -92,6 +92,7 @@ func SlackLinkFormatter(url, text string) string {
 
 // SlackLinkToRef slack-formatter link to a repo ref
 func SlackLinkToRef(repoURL, ref string) string {
+	// FIXME: SHA1 hardcoded here
 	url := git.RefURL(repoURL, ref)
 	refName := git.RefName(ref).ShortName()
 	return SlackLinkFormatter(url, refName)