travis-ci: retry if Git for Windows CI returns HTTP error 502 or 503

travis-ci: retry if Git for Windows CI returns HTTP error 502 or 503

The Git for Windows CI web app sometimes returns HTTP errors of
"502 bad gateway" or "503 service unavailable" [1]. We also need to
check the HTTP content because the GfW web app seems to pass through
(error) results from other Azure calls with HTTP code 200.
Wait a little and retry the request if this happens.

[1] https://docs.microsoft.com/en-in/azure/app-service-web/app-service-web-troubleshoot-http-502-http-503

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>