Git redirect silently ignored

[ben-edna]

During dfetch add, but also dfetch check and dfetch update

When a remote is determined if it is svn or git through subproject.check() under the hood ['git', 'ls-remote', '--heads', <url>] is done.

When the remote does a redirection because of for instance a sign-in that is required or other reason, this is silently ignored and assumed it is not git.

fatal: unable to update url base from redirection:
  asked for: http://git.my-company.com/some-repo/info/refs?service=git-upload-pack
   redirect: http://git.my-company.com/users/sign_in

We could see if we can detect (sign_in) redirects with actionable hints to user: "url seems to exist, but you need to sign in".