Update backport documentation to the subtree workflow

Author: flip1995

doc/backport.md

@@ -5,7 +5,7 @@ Backports in Clippy are rare and should be approved by the Clippy team. For
 example, a backport is done, if a crucial ICE was fixed or a lint is broken to a
 point, that it has to be disabled, before landing on stable.
 
-Backports are done to the `beta` release of Clippy. Backports to stable Clippy
+Backports are done to the `beta` branch of Clippy. Backports to stable Clippy
 releases basically don't exist, since this would require a Rust point release,
 which is almost never justifiable for a Clippy fix.
 
@@ -18,7 +18,31 @@ Backports are done on the beta branch of the Clippy repository.
 # Assuming the current directory corresponds to the Clippy repository
 $ git checkout beta
 $ git checkout -b backport
-$ git cherry-pick <SHA>  # `<SHA>` is the commit hash of the commit, that should be backported
+$ git cherry-pick <SHA>  # `<SHA>` is the commit hash of the commit(s), that should be backported
+$ git push origin backport
+```
+
+Now you should test that the backport passes all the tests in the Rust
+repository. You can do this with:
+
+```bash
+# Assuming the current directory corresponds to the Rust repository
+$ git checkout beta
+$ git subtree pull -p src/tools/clippy https://github.com/<your-github-name>/rust-clippy backport
+$ ./x.py test src/tools/clippy
+```
+
+Should the test fail, you can fix Clippy directly in the Rust repository. This
+has to be first applied to the Clippy beta branch and then again synced to the
+Rust repository, though. The easiest way to do this is:
+
+```bash
+# In the Rust repository
+$ git diff --patch --relative=src/tools/clippy > clippy.patch
+# In the Clippy repository
+$ git apply /path/to/clippy.patch
+$ git add -u
+$ git commit -m "Fix rustup fallout"
 $ git push origin backport
 ```
 
@@ -29,22 +53,19 @@ After this, you can open a PR to the `beta` branch of the Clippy repository.
 
 This step must be done, **after** the PR of the previous step was merged.
 
-After the backport landed in the Clippy repository, also the Clippy version on
-the Rust `beta` branch has to be updated.
+After the backport landed in the Clippy repository, the branch has to be synced
+back to the beta branch of the Rust repository.
 
 ```bash
 # Assuming the current directory corresponds to the Rust repository
 $ git checkout beta
 $ git checkout -b clippy_backport
-$ pushd src/tools/clippy
-$ git fetch
-$ git checkout beta
-$ popd
-$ git add src/tools/clippy
-§ git commit -m "Update Clippy"
+$ git subtree pull -p src/tools/clippy https://github.com/rust-lang/rust-clippy beta
 $ git push origin clippy_backport
 ```
 
-After this you can open a PR to the `beta` branch of the Rust repository. In
-this PR you should tag the Clippy team member, that agreed to the backport or
-the `@rust-lang/clippy` team. Make sure to add `[beta]` to the title of the PR.
+Make sure to test the backport in the Rust repository before opening a PR. This
+is done with `./x.py test src/tools/clippy`. If that passes all tests, open a PR
+to the `beta` branch of the Rust repository. In this PR you should tag the
+Clippy team member, that agreed to the backport or the `@rust-lang/clippy` team.
+Make sure to add `[beta]` to the title of the PR.