Add engine merge method configuration (#23953)

In order for this to take effect, https://skia-review.googlesource.com/c/buildbot/+/167200
needs to be landed and deployed.

The /bin/internal/engine.merge_method file controls how we merge a pull
request created by the engine auto-roller. If it's squash, there's only one
commit for a pull request no matter how many engine commits there are inside
that pull request. If it's rebase, the number of commits in the framework is
equal to the number of engine commits in the pull request. The latter method is
easier to detect regressions but cost more test resources.

bin/internal/README.md

diff --git a/bin/internal/README.md b/bin/internal/README.md
index da4e94f..c1634b1 100644
--- a/bin/internal/README.md
+++ b/bin/internal/README.md
@@ -4,3 +4,10 @@
 The `bin/internal/engine.version` file controls which version of the Flutter engine to use.
 The file contains the commit hash of a commit in the <https://github.com/flutter/engine> repository.
 That hash must have successfully been compiled on <https://build.chromium.org/p/client.flutter/> and had its artifacts (the binaries that run on Android and iOS, the compiler, etc) successfully uploaded to Google Cloud Storage.
+
+The `/bin/internal/engine.merge_method` file controls how we merge a pull
+request created by the engine auto-roller. If it's `squash`, there's only one
+commit for a pull request no matter how many engine commits there are inside
+that pull request. If it's `rebase`, the number of commits in the framework is
+equal to the number of engine commits in the pull request. The latter method
+makes it easier to detect regressions but costs more test resources.

bin/internal/engine.merge_method

diff --git a/bin/internal/engine.merge_method b/bin/internal/engine.merge_method
new file mode 100644
index 0000000..97ef86e
--- /dev/null
+++ b/bin/internal/engine.merge_method
@@ -0,0 +1 @@
+rebase