Merge branch 'jt/http-base-url-update-upon-redirect'

When a redirected http transport gets an error during the
redirected request, we ignored the error we got from the server,
and ended up giving a not-so-useful error message.

* jt/http-base-url-update-upon-redirect:
http: attempt updating base URL only if no error

diff --cc http.c
Simple merge

diff --cc t/t5550-http-fetch-dumb.sh
Simple merge