Debug to detect when default_branch is set in .tf file

WIP for fixing #299 where Terraform tries to reset the
`default_branch` to null after a repository is added on
GitLab side.

The `default_branch` defaults to `master` on GitLab side,
but it is only set if the repository is not empty.
Otherwise it is `null` on remote side.

The commit generates the following output, which shows
no trace of `null` value from remote.

    2020/06/19 20:55:55 [DEBUG] gitlab.Project default_branch (string) ""
    2020/06/19 20:55:55 [DEBUG] d.Get default_branch (string) ""
    2020/06/19 20:55:55 [DEBUG] d.GetOk default_branch (string) "" - false
    2020/06/19 20:55:55 [DEBUG] d.HasChange default_branch false
    2020/06/19 20:55:55 [DEBUG] d.GetChange default_branch (string) "", (string) ""

But the actual response shows that `null` was returned in
JSON.

    2020/06/19 20:55:55 [DEBUG] GitLab API Response Details:
    ...
     "default_branch": null,

Author: abitrolly

gitlab/resource_gitlab_project.go

@@ -213,6 +213,16 @@ func resourceGitlabProjectSetToState(d *schema.ResourceData, project *gitlab.Pro
 	d.Set("name", project.Name)
 	d.Set("path", project.Path)
 	d.Set("description", project.Description)
+
+	log.Printf("[DEBUG] gitlab.Project default_branch (%T) %#v", project.DefaultBranch, project.DefaultBranch)
+	default_branch := d.Get("default_branch")
+	db, ok := d.GetOk("default_branch")
+	changed := d.HasChange("default_branch")
+	old, new := d.GetChange("default_branch")
+	log.Printf("[DEBUG] d.Get default_branch (%T) %#v", default_branch, default_branch)
+	log.Printf("[DEBUG] d.GetOk default_branch (%T) %#v - %#v", db, db, ok)
+	log.Printf("[DEBUG] d.HasChange default_branch %#v", changed)
+	log.Printf("[DEBUG] d.GetChange default_branch (%T) %#v, (%T) %#v", old, old, new, new)
 	d.Set("default_branch", project.DefaultBranch)
 	d.Set("request_access_enabled", project.RequestAccessEnabled)
 	d.Set("issues_enabled", project.IssuesEnabled)