Expand tokens found in ssh config `HostName`

Per "TOKENS" section in ssh_config(5)

Fixes #1936

git/ssh_config.go

@@ -61,7 +61,7 @@ func (r *SSHConfigReader) readFile(c SSHConfig, re *regexp.Regexp, f string) err
 		} else {
 			for _, host := range hosts {
 				for _, name := range names {
-					c[host] = name
+					c[host] = expandTokens(name, host)
 				}
 			}
 		}
@@ -69,3 +69,8 @@ func (r *SSHConfigReader) readFile(c SSHConfig, re *regexp.Regexp, f string) err
 
 	return scanner.Err()
 }
+
+func expandTokens(text, host string) string {
+	expanded := strings.Replace(text, "%h", host, -1)
+	return strings.Replace(expanded, "%%", "%", -1)
+}

git/ssh_config_test.go

@@ -24,3 +24,17 @@ func TestSSHConfigReader_Read(t *testing.T) {
 	sc := r.Read()
 	assert.Equal(t, "ssh.github.com", sc["github.com"])
 }
+
+func TestSSHConfigReader_ExpandTokens(t *testing.T) {
+	f, _ := ioutil.TempFile("", "ssh-config")
+	c := `Host github.com example.org
+  Hostname 1-%h-2-%%-3-%h-%%
+	`
+
+	ioutil.WriteFile(f.Name(), []byte(c), os.ModePerm)
+
+	r := &SSHConfigReader{[]string{f.Name()}}
+	sc := r.Read()
+	assert.Equal(t, "1-github.com-2-%-3-github.com-%", sc["github.com"])
+	assert.Equal(t, "1-example.org-2-%-3-example.org-%", sc["example.org"])
+}