[Downloader]: RepoManager: don't load repos in __init__ (#2829)

Loading repos is already done in initialize() method.
This could actually turn out badly
if both
of git processes would touch the same repo at the same time.

This also fixes create_backup in
setup.py - now it properly generates repos.json

redbot/setup.py

@@ -409,8 +409,9 @@ async def create_backup(instance):
             from redbot.cogs.downloader.repo_manager import RepoManager
 
             repo_mgr = RepoManager()
+            await repo_mgr.initialize()
             repo_output = []
-            for _, repo in repo_mgr._repos:
+            for repo in repo_mgr._repos.values():
                 repo_output.append({"url": repo.url, "name": repo.name, "branch": repo.branch})
             repo_filename = pth / "cogs" / "RepoManager" / "repos.json"
             with open(str(repo_filename), "w") as f: