Prepare zinc repo for moving bridge to scala/scala

```
git clone git@github.com:sbt/zinc.git zinc-bridge-only

➜ zinc-bridge-only git log --oneline -n1 | cat
dfdb20994 Update scalafmt-core to 3.7.8
```

Filter-branch to move bridge sources to `internal/compiler-bridge`
across the entire history (they were in `internal/compile-bridge` and
`compile/interface` in early history). Add a reference to the original
zinc commit to every commit message.

Then filter-branch to keep only `internal/compiler-bridge`. This also
removes merge commits.

```
➜ zinc-bridge-only git filter-branch -f --tree-filter 'if [ -d internal/compile-bridge ]; then mv internal/compile-bridge internal/compiler-bridge; elif [ -d compile/interface ]; then mkdir -p internal; mv compile/interface internal/compiler-bridge; fi' --msg-filter 'cat; echo; echo Rewritten from sbt/zinc@$GIT_COMMIT' bridge-only

➜ zinc-bridge-only git filter-branch -f --prune-empty --subdirectory-filter internal/compiler-bridge bridge-only
```

Clean out the repo.

```
➜ zinc-bridge-only git clean -f -d
➜ zinc-bridge-only for t in `git tag`; do git tag -d $t; done
➜ zinc-bridge-only git br -D develop
➜ zinc-bridge-only git remote remove origin
➜ zinc-bridge-only git gc --prune=now

➜ zinc-bridge-only git rm -r src/main/scala_2.10
➜ zinc-bridge-only git rm -r src/main/scala-2.11
➜ zinc-bridge-only git rm -r src/main/scala-2.12
➜ zinc-bridge-only git rm -r src/main/scala_2.11-12
```

Move bridge sources to `src/sbt-bridge`

```
➜ zinc-bridge-only mkdir -p src/sbt-bridge/scala/tools/xsbt/
➜ zinc-bridge-only git mv src/main/scala/xsbt/* src/sbt-bridge/scala/tools/xsbt/
➜ zinc-bridge-only git mv src/main/scala_2.13/xsbt/* src/sbt-bridge/scala/tools/xsbt/
➜ zinc-bridge-only git mv src/main/scala_2.13/scala/ZincCompat.scala src/sbt-bridge/scala/tools/xsbt
➜ zinc-bridge-only git mv src/main/resources src/sbt-bridge/

➜ zinc-bridge-only rm -rf src/main
```