fix(oauth): don't wait for the browser opener to exit (#1093)

Fixes `tea login add --oauth` hanging after the user authenticates in the
browser.

`xdg-open` (at least on Debian) runs the browser in the foreground, so it does
not exit until the browser does. `open.Run` waits for it, so tea is blocked and
doesn't get the oAuth callback from the browser.

This only happens when `xdg-open` has to start the browser. With one already
running, the new process hands off and exits immediately.

`open.Start` launches the opener and returns. The test mocks `xdg-open` with a
script that holds the foreground and fails if `openBrowser` waits on it.

Reviewed-on: https://gitea.com/gitea/tea/pulls/1093
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: James Braid <jamesb@loreland.org>

modules/auth/oauth.go

@@ -366,7 +366,9 @@ func startLocalServerAndOpenBrowser(authURL, expectedState string, opts *OAuthOp
 var openBrowser = func(url string) error {
 	fmt.Printf("Please authorize the application by visiting this URL in your browser:\n%s\n", url)
 
-	return open.Run(url)
+	// Don't wait for the opener to exit, so a browser that holds the
+	// foreground can't block the wait for the callback.
+	return open.Start(url)
 }
 
 // createLoginFromToken creates a login entry using the obtained access token