hcp: don't continue if HEAD doesn't exist

In HCP's metadata package, especially the VCS/git parts, we keep the
current HEAD for a repository, along with the state it is in, in order
to report it to HCP Packer when the build completes.

However, when a build is run on a template from an empty Git repository,
and HCP Packer is enabled, the code would crash when trying to get the
information on the current HEAD, as it doesn't exist.

The git library we use returns an error in such a case, but this was
ignored, leading to a crash when attempting to get the hash to this
reference later on.

This commit fixes the problem by NOT ignoring the error to get the head,
and immediately stop processing the git data as it doesn't yet exist.

internal/hcp/registry/metadata/vcs.go

@@ -53,13 +53,14 @@ func (g *Git) Type() string {
 }
 
 func (g *Git) Details() map[string]interface{} {
-	resp := map[string]interface{}{}
-
 	headRef, err := g.repo.Head()
 	if err != nil {
-		log.Printf("[ERROR] failed to get the git branch name: %s", err)
-	} else {
-		resp["ref"] = headRef.Name().Short()
+		log.Printf("[ERROR] failed to get reference to git HEAD: %s", err)
+		return nil
+	}
+
+	resp := map[string]interface{}{
+		"ref": headRef.Name().Short(),
 	}
 
 	commit, err := g.repo.CommitObject(headRef.Hash())