Fix deleteRepos in mirror-github

When support for github enterprise was added (5208c28),
deleting stale repos for github.com broke.

`baseURL` is set to something like `https://github.com/foouser/barrepo`.

Doing `url.Parse(baseURL + user)` would then result in
`https://github.com/foouser/barrepofoouser`, which clearly does not
work.

To fix this, we now parse the baseURL and then modify the path to be the
user, resulting in the correct URL: `https://github.com/foouser`.

Change-Id: I1df0bfa214689e2ecc009878961ce27f6f178509
diff --git a/cmd/zoekt-mirror-github/main.go b/cmd/zoekt-mirror-github/main.go
index 7973362..1131a28 100644
--- a/cmd/zoekt-mirror-github/main.go
+++ b/cmd/zoekt-mirror-github/main.go
@@ -161,10 +161,11 @@
 	} else {
 		return nil
 	}
-	u, err := url.Parse(baseURL + user)
+	u, err := url.Parse(baseURL)
 	if err != nil {
 		return err
 	}
+	u.Path = user
 
 	paths, err := gitindex.ListRepos(destDir, u)
 	if err != nil {