Fix URL building for default repo branch API

Using the base URL in path.Join caused it to remove the two slashes (because /////tmp == /tmp):

`https:/tildegit.org/api/v1/repos/owner/name`

Then, using that in URL.Parse causes it to see a `https:` scheme, and the / right afterwards means "start a path here", not a hostname, because hostnames begin with two slashes. That's a valid URL, with no hostname. Then it gets turned back into a string:

`https:///tildegit.org/api/v1/repos/owner/name`

That's another way of expressing the same URL with no hostname: //<empty hostname>/<path>.

curl, Firefox and Chromium all fix the triple slash automatically, but fasthttp doesn't, and it tries to look up the empty string as a DNS host!

server/upstream/gitea.go

@@ -64,7 +64,7 @@ func giteaGetRepoDefaultBranch(giteaRoot, repoOwner, repoName, giteaAPIToken str
 
 	req := fasthttp.AcquireRequest()
 	baseUrl, _ := url.Parse(giteaRoot)
-	apiPath, _ := baseUrl.Parse(path.Join(giteaRoot, giteaAPIRepos, repoOwner, repoName))
+	apiPath, _ := baseUrl.Parse(path.Join(giteaAPIRepos, repoOwner, repoName))
 
 	log.Debug().Msgf("giteaGetRepoDefaultBranch: trying path %s", apiPath)
 	req.SetRequestURI(apiPath.String())