Implement the action (#1)

LICENSE

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+Copyright (c) 2021, Google LLC
+
+Permission is hereby granted, free of charge, to any person obtaining
+a copy of this software and associated documentation files (the
+"Software"), to deal in the Software without restriction, including
+without limitation the rights to use, copy, modify, merge, publish,
+distribute, sublicense, and/or sell copies of the Software, and to
+permit persons to whom the Software is furnished to do so, subject to
+the following conditions:
+
+The above copyright notice and this permission notice shall be
+included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+This is a small utility action that clones a repository, using a ref linked from
+a pull request if possible.
+
+For example:
+
+```yaml
+jobs:
+  sass-spec:
+    name: Language Tests
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+      matrix:
+        dart_channel: [stable, dev]
+        async_label: [synchronous]
+        async_args: ['']
+        include:
+          - dart_channel: stable
+            async_label: asynchronous
+            async_args: '--cmd-args --async'
+
+    steps:
+      - uses: actions/checkout@v2
+      - uses: dart-lang/setup-dart@v1
+        with: {sdk: stable}
+      - uses: actions/setup-node@v2
+        with: {node-version: 16}
+      - run: dart pub get
+
+      - name: Check out sass-spec
+        uses: sass/clone-linked-repo@v1
+        args: {repo: sass/sass-spec}
+      - run: npm install
+        working-directory: sass-spec
+      - name: Run specs
+        run: npm run sass-spec -- --dart .. $extra_args
+        working-directory: sass-spec
+        env: {extra_args: "${{ matrix.async_args }}"}
+```
+
+By default, this will clone `sass-spec`'s `main` branch into the `sass-spec`
+path. But if a PR contains the text "sass/sass-spec#123" and there's a pull
+request with ID 123, it will clone `pull/123/head` instead.
+
+This action is meant primarily for Sass repositories. Other repositories are
+welcome to use it, but please be aware that support will be a very low
+priority.
+
+## Details
+
+* This will recognize both short references (`sass/sass-spec#123`) and full URLs
+  (`https://github.com/sass/sass-spec/pulls/123`).
+
+* This will ignore issue references.
+
+* If the PR message contains multiple references to a PR from the same
+  repository, this will use the first one.

action.yaml

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+name: Cloned Linked Repo
+description: Clone a repo at a version linked from a PR description.
+
+inputs:
+  repo:
+    description: >
+      The slug (e.g. "sass/sass") of the repository whose PR links this action
+      should look for.
+    required: true
+  token:
+    description: >
+      The GitHub token used to access the pull request's message.
+    default: ${{ github.token }}
+  default-ref:
+    description: >
+      The default branch to check out when this isn't a PR or doesn't link to a
+      PR. Note that if the current branch or PR base is of the form "feature.*",
+      this will always use the same branch name as the default.
+    default: main
+  ssh-key:
+    description: Passed directly to actions/checkout.
+  ssh-known-hosts:
+    description: Passed directly to actions/checkout.
+  ssh-strict:
+    description: Passed directly to actions/checkout.
+  persist-credentials:
+    description: Passed directly to actions/checkout.
+  path:
+    description: Passed directly to actions/checkout.
+  clean:
+    description: Passed directly to actions/checkout.
+  fetch-depth:
+    description: Passed directly to actions/checkout.
+  lfs:
+    description: Passed directly to actions/checkout.
+  submodules:
+    description: Passed directly to actions/checkout.
+
+runs:
+  using: composite
+  steps:
+
+  - run: "bash $GITHUB_ACTION_PATH/find-ref.sh"
+    id: find-ref
+    shell: bash
+    env:
+      PR_BRANCH: ${{ github.base_ref }}
+      CURRENT_REF: ${{ github.ref }}
+      PR_BODY: ${{ github.event.pull_request.body }}
+      # Inputs must be repeated due to actions/runner#665
+      REPO: ${{ inputs.repo }}
+      TOKEN: ${{ inputs.token }}
+      DEFAULT_REF: ${{ inputs.default-ref }}
+
+  - uses: actions/checkout@v2
+    with:
+      repository: ${{ inputs.repo }}
+      ref: ${{ steps.find-ref.outputs.ref }}
+      token: ${{ inputs.token }}
+      ssh-key: ${{ inputs.ssh-key }}
+      ssh-known-hosts: ${{ inputs.ssh-known-hosts }}
+      ssh-strict: ${{ inputs.ssh-strict }}
+      persist-credentials: ${{ inputs.persist-credentials }}
+      path: ${{ inputs.path }}
+      clean: ${{ inputs.clean }}
+      fetch-depth: ${{ inputs.fetch-depth }}
+      lfs: ${{ inputs.lfs }}
+      submodules: ${{ inputs.submodules }}

find-ref.sh

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+#!/bin/bash -e
+# Copyright 2021 Google Inc. Use of this source code is governed by an MIT-style
+# license that can be found in the LICENSE file or at
+# https://opensource.org/licenses/MIT.
+
+# Echoes the sass/sass Git ref that should be checked out for the current GitHub
+# Actions run. If we're running specs for a pull request which refers to a
+# sass/sass pull request, we'll run against the latter rather than sass/sass
+# main.
+if [ -z "$PR_BRANCH" ]; then
+  # Remove the "refs/heads/" prefix
+  current_branch="${CURRENT_REF/refs\/heads\//}"
+else
+  current_branch="$PR_BRANCH"
+fi
+
+if [[ "$current_branch" == feature.* ]]; then
+  default="$current_branch"
+else
+  default="$DEFAULT_REF"
+fi
+
+# We don't have a PR_BRANCH so we are not in a pull request, so there's no
+# linked PR to find.
+if [ -z "$PR_BRANCH" ]; then
+  echo "::set-output name=ref::$default"
+  exit 0
+fi
+
+echo "::group::Pull request body"
+echo "$PR_BODY"
+echo "::endgroup::"
+
+echo "::group::Finding pull request reference"
+for link in "$(echo "$PR_BODY" | grep -Eo "${REPO}(#|/pull/)[0-9]+")"; do
+  if [[ "$link" = *#* ]]; then
+    number="${link#*#}"
+  else
+    number="${link#*/pull/}"
+  fi
+
+  if curl --fail --silent --head --header "Authorization: token $TOKEN" \
+       "https://api.github.com/repos/$REPO/pulls/${number}" > /dev/null; then
+    echo "Linked to pull request $number"
+    echo "::set-output name=ref::refs/pull/$number/head"
+    exit 0
+  else
+    echo "$link isn't a pull request."
+  fi
+done
+
+echo "No linked pull request, using default ref $default"
+echo "::set-output name=ref::$default"
+echo "::endgroup::"