Regression in 0.4.12 when using private gitlab

[jocelynthode]

Hey,

While upgrading to the 0.4.12 version, I noticed that the feature introduced in #366 specifically in commit ba04564

The problem appears at this line: ba04564#diff-4431759fdf270c83e7713adcd0cf7d5aR52

We use URL.Parse() on a domain which does not create an error but returns an empty string when calling Hostname() afterwards.

I would suggest to either rename the parameter GitlabHostname to GitlabURL and use a URL from now on or jsut before calling the URL parse, format the string to contain the scheme so that we can parse it correctly.
I would also add a unit test to verify that this continue to work in the future.

@lkysow I would gladly submit a PR for this issue once we have worked out what's the best solution if that's ok with you.

P.S.: Here is an example code to reproduce the issue:

package main

import (
	"fmt"
	"log"
	"net/url"
)

func main() {
	u, err := url.Parse("https://example.org")
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(u.Hostname())
	
	u, err = url.Parse("example.org")
	if err != nil {
		log.Fatal(err)
	}
	fmt.Println(u.Hostname())

}

Output:

example.org

[lkysow]

Ahh good catch. I'm thinking something like:

// If not using gitlab.com we need to set the URL to the API.
if hostname != "gitlab.com" {
	if !strings.HasPrefix(hostname, "http://") && !strings.HasPrefix(hostname, "https://") {
		hostname = "https://" + hostname
	}

	// Warn if this hostname isn't resolvable. The GitLab client
	// doesn't give good error messages in this case.
	u, err := url.Parse(hostname)
	if err != nil {
		return nil, errors.Wrapf(err, "parsing hostname %q", hostname)
	}
	ips, err := net.LookupIP(u.Hostname())
	if err != nil {
		logger.Warn("unable to resolve %q: %s", u.Hostname(), err)
	} else if len(ips) == 0 {
		logger.Warn("found no IPs while resolving %q", u.Hostname())
	}

	// Now we're ready to construct the client.
	hostname = strings.TrimSuffix(hostname, "/")
	apiURL := fmt.Sprintf("%s/api/v4/", hostname)
	if err := client.Client.SetBaseURL(apiURL); err != nil {
		return nil, errors.Wrapf(err, "setting GitLab API URL: %s", apiURL)
	}
}