Use curl for downloads from git remotes when annex.url-options and other
git configs are set.

If the url needs a password, curl will fail, and git credential will not be
used to prompt for it. But the user can set --netrc in url-options and
put the password in the netrc file.

This also means that url-options settings like -4 will take effect.
That was the case before commit 1883f7ef
forced conduit to be used.