use the correct corresponding subtree directory and remote repository.

The synchronization process goes in two directions: `subtree push` and `subtree pull`.

A `subtree push` takes all the changes that happened to the copy in this repo and creates commits
on the remote repo that match the local changes. Every local
commit that touched the subtree causes a commit on the remote repo, but
is modified to move the files from the specified directory to the tool repo root.

A `subtree pull` takes all changes since the last `subtree pull`
from the tool repo and adds these commits to the rustc repo along with a merge commit that moves
the tool changes into the specified directory in the Rust repository.

It is recommended that you always do a push first and get that merged to the tool master branch.
Then, when you do a pull, the merge works without conflicts.
While it's definitely possible to resolve conflicts during a pull, you may have to redo the conflict
resolution if your PR doesn't get merged fast enough and there are new conflicts. Do not try to
rebase the result of a `git subtree pull`, rebasing merge commits is a bad idea in general.

You always need to specify the `-P` prefix to the subtree directory and the corresponding remote
repository. If you specify the wrong directory or repository
you'll get very fun merges that try to push the wrong directory to the wrong remote repository.
Luckily you can just abort this without any consequences by throwing away either the pulled commits
in rustc or the pushed branch on the remote and try again. It is usually fairly obvious
that this is happening because you suddenly get thousands of commits that want to be synchronized.


### Creating a new subtree dependency

If you want to create a new subtree dependency from an existing repository, call (from this
repository's root directory!)

```
git subtree add -P src/tools/clippy https://github.com/rust-lang/rust-clippy.git master
```

This will create a new commit, which you may not rebase under any circumstances! Delete the commit
and redo the operation if you need to rebase.

Now you're done, the `src/tools/clippy` directory behaves as if Clippy were
part of the rustc monorepo, so no one but you (or others that synchronize
subtrees) actually needs to use `git subtree`.

## External Dependencies (submodules)

Building Rust will also use external git repositories tracked using [git
submodules]. The complete list may be found in the [`.gitmodules`] file. Some
of these projects are required (like `stdarch` for the standard library) and
some of them are optional (like `src/doc/book`).

Usage of submodules is discussed more in the [Using Git chapter](git.md#git-submodules).

Some of the submodules are allowed to be in a "broken" state where they
either don't build or their tests don't pass, e.g. the documentation books
like [The Rust Reference]. Maintainers of these projects will be notified
when the project is in a broken state, and they should fix them as soon
as possible. The current status is tracked on the [toolstate website].
More information may be found on the Forge [Toolstate chapter].
In practice, it is very rare for documentation to have broken toolstate.

Breakage is not allowed in the beta and stable channels, and must be addressed
before the PR is merged. They are also not allowed to be broken on master in
the week leading up to the beta cut.