Resolve SSH host aliases by reading from `~/.ssh/config`

This allows someone to have something like this in their SSH config:

    Host gh
      HostName github.com

And then use git remote URLs such as:

    gh:myname/myrepo.git

Since git itself allows people to do this, we need to support it in hub.

git/url.go

 package git
 
 import (
-	"fmt"
+	"bufio"
 	"net/url"
+	"os"
+	"path/filepath"
 	"regexp"
+	"strings"
+)
+
+var (
+	ProtocolRe     = regexp.MustCompile("^[a-zA-Z_-]+://")
+	SshConfigFiles = []string{
+		filepath.Join(os.Getenv("HOME"), ".ssh/config"),
+		"/etc/ssh_config",
+		"/etc/ssh/ssh_config",
+	}
+	SshConfig map[string]string
 )
 
 func ParseURL(rawurl string) (u *url.URL, err error) {
-	sshGitRegexp := regexp.MustCompile(`(.+)@(.+):(.+)(\.git)?`)
-	if sshGitRegexp.MatchString(rawurl) {
-		match := sshGitRegexp.FindStringSubmatch(rawurl)
-		user := match[1]
-		host := match[2]
-		path := match[3]
-		ext := match[4]
-		rawurl = fmt.Sprintf("ssh://%s@%s/%s%s", user, host, path, ext)
+	if !ProtocolRe.MatchString(rawurl) && strings.Contains(rawurl, ":") {
+		rawurl = "ssh://" + strings.Replace(rawurl, ":", "/", 1)
+	}
+
+	u, err = url.Parse(rawurl)
+	if err == nil {
+		if SshConfig == nil {
+			SshConfig = readSshConfig()
+		}
+		if SshConfig[u.Host] != "" {
+			u.Host = SshConfig[u.Host]
+		}
+	}
+	return
+}
+
+func readSshConfig() map[string]string {
+	config := make(map[string]string)
+	hostRe := regexp.MustCompile("^[ \t]*(Host|HostName)[ \t]+(.+)$")
+
+	for _, filename := range SshConfigFiles {
+		file, err := os.Open(filename)
+		if err != nil {
+			continue
+		}
+		hosts := []string{"*"}
+		scanner := bufio.NewScanner(file)
+		for scanner.Scan() {
+			line := scanner.Text()
+			match := hostRe.FindStringSubmatch(line)
+			if match == nil {
+				continue
+			}
+			names := strings.Fields(match[2])
+			if match[1] == "Host" {
+				hosts = names
+			} else {
+				for _, host := range hosts {
+					for _, name := range names {
+						config[host] = name
+					}
+				}
+			}
+		}
+		file.Close()
 	}
 
-	return url.Parse(rawurl)
+	return config
 }