Move subtree part of Readme into its own doc file

Author: GuillaumeGomez

Readme.md

@@ -269,51 +269,6 @@ COLLECT_NO_DEMANGLE=1
  * Build the stage2 compiler (`rustup toolchain link debug-current build/x86_64-unknown-linux-gnu/stage2`).
  * Clean and rebuild the codegen with `debug-current` in the file `rust-toolchain`.
 
-### How to install a forked git-subtree
-
-Using git-subtree with `rustc` requires a patched git to make it work.
-The PR that is needed is [here](https://github.com/gitgitgadget/git/pull/493).
-Use the following instructions to install it:
-
-```bash
-git clone [email protected]:tqc/git.git
-cd git
-git checkout tqc/subtree
-make
-make install
-cd contrib/subtree
-make
-cp git-subtree ~/bin
-```
-
-Then, do a sync with this command:
-
-```bash
-PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
-cd ../rustc_codegen_gcc
-git checkout master
-git pull
-git checkout sync_branch_name
-git merge master
-```
-
-To send the changes to the rust repo:
-
-```bash
-cd ../rust
-git pull origin master
-git checkout -b subtree-update_cg_gcc_YYYY-MM-DD
-PATH="$HOME/bin:$PATH" ~/bin/git-subtree pull --prefix=compiler/rustc_codegen_gcc/ https://github.com/rust-lang/rustc_codegen_gcc.git master
-git push
-
-# Immediately merge the merge commit into cg_gcc to prevent merge conflicts when syncing from rust-lang/rust later.
-PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
-```
-
-TODO: write a script that does the above.
-
-https://rust-lang.zulipchat.com/#narrow/stream/301329-t-devtools/topic/subtree.20madness/near/258877725
-
 ### How to use [mem-trace](https://github.com/antoyo/mem-trace)
 
 `rustc` needs to be built without `jemalloc` so that `mem-trace` can overload `malloc` since `jemalloc` is linked statically, so a `LD_PRELOAD`-ed library won't a chance to intercept the calls to `malloc`.

doc/subtree.md

@@ -0,0 +1,52 @@
+# git subtree sync
+
+`rustc_codegen_gcc` is a subtree of the rust compiler. As such, it needs to be
+sync from time to time to ensure changes that happened on their side are also
+included on our side.
+
+### How to install a forked git-subtree
+
+Using git-subtree with `rustc` requires a patched git to make it work.
+The PR that is needed is [here](https://github.com/gitgitgadget/git/pull/493).
+Use the following instructions to install it:
+
+```bash
+git clone [email protected]:tqc/git.git
+cd git
+git checkout tqc/subtree
+make
+make install
+cd contrib/subtree
+make
+cp git-subtree ~/bin
+```
+
+### Syncing with rust compiler
+
+Do a sync with this command:
+
+```bash
+PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
+cd ../rustc_codegen_gcc
+git checkout master
+git pull
+git checkout sync_branch_name
+git merge master
+```
+
+To send the changes to the rust repo:
+
+```bash
+cd ../rust
+git pull origin master
+git checkout -b subtree-update_cg_gcc_YYYY-MM-DD
+PATH="$HOME/bin:$PATH" ~/bin/git-subtree pull --prefix=compiler/rustc_codegen_gcc/ https://github.com/rust-lang/rustc_codegen_gcc.git master
+git push
+
+# Immediately merge the merge commit into cg_gcc to prevent merge conflicts when syncing from rust-lang/rust later.
+PATH="$HOME/bin:$PATH" ~/bin/git-subtree push -P compiler/rustc_codegen_gcc/ ../rustc_codegen_gcc/ sync_branch_name
+```
+
+TODO: write a script that does the above.
+
+https://rust-lang.zulipchat.com/#narrow/stream/301329-t-devtools/topic/subtree.20madness/near/258877725