More stability checker options (#5299)

## Summary

This contains three changes:
* repos in `check_ecosystem.py` are stored as `org:name` instead of
`org/name` to create a flat directory layout
* `check_ecosystem.py` performs a maximum of 50 parallel jobs at the
same time to avoid consuming to much RAM
* `check-formatter-stability` gets a new option `--multi-project` so
it's possible to do `cargo run --bin ruff_dev --
check-formatter-stability --multi-project target/checkouts`
With these three changes it becomes easy to check the formatter
stability over a larger number of repositories. This is part of the
integration of integrating formatter regressions checks into the
ecosystem checks.

## Test Plan

```shell
python scripts/check_ecosystem.py --checkouts target/checkouts --projects github_search.jsonl -v $(which true) $(which true)
cargo run --bin ruff_dev -- check-formatter-stability --multi-project target/checkouts
```

scripts/check_ecosystem.py

@@ -44,11 +44,11 @@ class Repository(NamedTuple):
     async def clone(self: Self, checkout_dir: Path) -> AsyncIterator[Path]:
         """Shallow clone this repository to a temporary directory."""
         if checkout_dir.exists():
-            logger.debug(f"Reusing {self.org}/{self.repo}")
+            logger.debug(f"Reusing {self.org}:{self.repo}")
             yield Path(checkout_dir)
             return
 
-        logger.debug(f"Cloning {self.org}/{self.repo}")
+        logger.debug(f"Cloning {self.org}:{self.repo}")
         git_command = [
             "git",
             "clone",
@@ -177,18 +177,17 @@ async def compare(
     """Check a specific repository against two versions of ruff."""
     removed, added = set(), set()
 
-    # Allows to keep the checkouts locations
+    # By the default, the git clone are transient, but if the user provides a
+    # directory for permanent storage we keep it there
     if checkouts:
-        checkout_parent = checkouts.joinpath(repo.org)
-        # Don't create the repodir itself, we need that for checking for existing
-        # clones
-        checkout_parent.mkdir(exist_ok=True, parents=True)
-        location_context = nullcontext(checkout_parent)
+        location_context = nullcontext(checkouts)
     else:
         location_context = tempfile.TemporaryDirectory()
 
     with location_context as checkout_parent:
-        checkout_dir = Path(checkout_parent).joinpath(repo.repo)
+        assert ":" not in repo.org
+        assert ":" not in repo.repo
+        checkout_dir = Path(checkout_parent).joinpath(f"{repo.org}:{repo.repo}")
         async with repo.clone(checkout_dir) as path:
             try:
                 async with asyncio.TaskGroup() as tg:
@@ -284,8 +283,19 @@ async def main(
 
     logger.debug(f"Checking {len(repositories)} projects")
 
+    # https://stackoverflow.com/a/61478547/3549270
+    # Otherwise doing 3k repositories can take >8GB RAM
+    semaphore = asyncio.Semaphore(50)
+
+    async def limited_parallelism(coroutine):  # noqa: ANN
+        async with semaphore:
+            return await coroutine
+
     results = await asyncio.gather(
-        *[compare(ruff1, ruff2, repo, checkouts) for repo in repositories.values()],
+        *[
+            limited_parallelism(compare(ruff1, ruff2, repo, checkouts))
+            for repo in repositories.values()
+        ],
         return_exceptions=True,
     )
 
@@ -433,6 +443,8 @@ if __name__ == "__main__":
         logging.basicConfig(level=logging.INFO)
 
     loop = asyncio.get_event_loop()
+    if args.checkouts:
+        args.checkouts.mkdir(exist_ok=True, parents=True)
     main_task = asyncio.ensure_future(
         main(
             ruff1=args.ruff1,