Support 307 redirects using relative Location headers

Relative Location headers are valid and are more convenient for some
servers which aren't aware of the original request (e.g. load balancers)
without checking for other headers which can vary per routing system.
git-lfs should just support relative Location responses.

lfs/client.go

@@ -9,6 +9,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net/http"
+	"net/url"
 	"os"
 	"path/filepath"
 	"regexp"
@@ -374,7 +375,15 @@ func doApiRequestWithRedirects(req *http.Request, creds Creds, via []*http.Reque
 	}
 
 	if res.StatusCode == 307 {
-		redirectedReq, redirectedCreds, err := newClientRequest(req.Method, res.Header.Get("Location"))
+		redirectTo := res.Header.Get("Location")
+		locurl, err := url.Parse(redirectTo)
+		if err == nil && !locurl.IsAbs() {
+			// relative Location headers are allowed & should be supported
+			locurl = req.URL.ResolveReference(locurl)
+			redirectTo = locurl.String()
+		}
+
+		redirectedReq, redirectedCreds, err := newClientRequest(req.Method, redirectTo)
 		if err != nil {
 			return res, Errorf(err, err.Error())
 		}