middleware: fix handling of Git 'info/refs' command to give correct access control

For a pull, the Git client first sends an 'info/refs' command with a
'service=git-upload-pack' query, then it sends the actual 'git-upload-pack'
command.

For a push, the Git client first sends an 'info/refs' command with a
'service=git-receive-pack' query, then it sends the actual 'git-receive-pack'
command.

Before, the 'info/refs' commands would fall back to the default of trying to
use the action of the previous request. That seems wrong.

Instead, authorize the 'info/refs' command just like the actual command it
references.

path_info will now be checked more than before. Mainly because that is more
correct and more explicit and "better" to do it that way. It might also give
some safety.