Discovered while debugging #174155, which is why recipes/+/68803 has not been reverted yet.

@jtmcdole and I accidentally discovered that release_builder.py/GetRepoInfo is wrong - it always fetches (and implicitly, checks out) the HEAD of the given release branch, not the current commit. This causes inconsistent builds if there are multiple cherrypicks merged.

Imagine the following, where we've merged 3 cherrypicks into flutter-3.36-candidate.0:

⬜ abc104 -- HEAD of flutter-3.36-candidate.0
⬜ abc103
🟨 abc102
🟩 abc101

When running a (release) engine (and docs? I think?) build for abc102, we do the following:

1. Initialize Git (OK)

Executing command [
  '/b/s/w/ir/cipd_bin_packages/cpython3/bin/python3',
  '-u',
  '/b/s/w/ir/kitchen-checkout/depot_tools/recipes/recipe_modules/git/resources/git_setup.py',
  '--path',
  '/b/s/w/ir/x/w/mirrors/flutter',
  '--url',
  'https://flutter.googlesource.com/mirrors/flutter',
]

2. Fetch the HEAD of the branch:

Executing command [
  '/b/s/w/ir/cipd_bin_packages/git',
  'fetch',
  'origin',
  'flutter-3.35-candidate.0',
  '--recurse-submodules',
  '--progress',
  '--tags',
]

3. Checkout the HEAD of the branch (Wrong!)

Executing command [
  '/b/s/w/ir/cipd_bin_packages/git',
  'checkout',
  '-f',
  'FETCH_HEAD',
]

That means, in the above example, abc102 checks out (and builds) abc104.

There seems to be a conflation between the term git_ref - it sometimes (elsewhere) means "the current SHA" (and indeed, even in the recipe it's documented as * The git ref to check out) - but in this recipe it's actually returning the branch name, and indeed that is what the gitiles data that is being passed to the builder contains:

{
  "$recipe_engine/scheduler": {
    "hostname": "luci-scheduler.appspot.com",
    "invocation": "8871440901813659728",
    "job": "dart-internal/Linux flutter_release_builder",
    "triggers": [
      {
        "gitiles": {
          "ref": "refs/heads/flutter-3.35-candidate.0",
          "repo": "https://flutter.googlesource.com/mirrors/flutter",
          "revision": "03e97fb327ab9c9ebdb0d0928e4735cdc7c7c0e2"
        },