Add workflow to automate manpage updates

.github/workflows/update-manpage.yml

@@ -0,0 +1,75 @@
+name: Update manpage and completions
+
+on:
+  push:
+    paths:
+      - .github/workflows/update-manpage.yml
+      - README.md
+      - Library/Homebrew/cmd/**
+      - Library/Homebrew/dev-cmd/**
+      - Library/Homebrew/completions/**
+      - Library/Homebrew/manpages/**
+      - Library/Homebrew/cli/parser.rb
+      - Library/Homebrew/env_config.rb
+    branches:
+      - master
+  schedule:
+    - cron: "0 0 * * *"
+  workflow_dispatch:
+
+jobs:
+  update-manpage:
+    runs-on: ubuntu-latest
+    if: github.repository == 'Homebrew/brew'
+    steps:
+      - name: Setup Homebrew
+        uses: Homebrew/actions/setup-homebrew@master
+
+      - name: Configure Git user
+        uses: Homebrew/actions/git-user-config@master
+        with:
+          username: BrewTestBot
+
+      - name: Update manpage and completions
+        id: update
+        run: |
+          git fetch origin
+
+          BRANCH=update-manpage
+          echo "::set-output name=branch::${BRANCH}"
+
+          if git ls-remote --exit-code --heads origin "$BRANCH"; then
+            git checkout "$BRANCH"
+            git reset origin/master
+          else
+            git checkout -B "$BRANCH" origin/master
+            BRANCH_EXISTS="1"
+          fi
+
+          brew man
+
+          if [ -n "$(git status --porcelain=v1 2>/dev/null)" ]; then
+            git add "$GITHUB_WORKSPACE/docs/Manpage.md" \
+                    "$GITHUB_WORKSPACE/manpages/brew.1" \
+                    "$GITHUB_WORKSPACE/completions"
+            git commit -m "Update manpage and completions." \
+                       -m "Autogenerated by the [update-manpage](https://github.com/Homebrew/brew/blob/HEAD/.github/workflows/update-manpage.yml) workflow."
+            echo "::set-output name=committed::true"
+            if [ -n "$BRANCH_EXISTS" ]; then
+              echo "::set-output name=pull_request::true"
+            fi
+          fi
+
+      - name: Push commits
+        if: steps.update.outputs.committed == 'true'
+        uses: Homebrew/actions/git-try-push@master
+        with:
+          token: ${{ secrets.HOMEBREW_GITHUB_API_TOKEN }}
+          branch: ${{ steps.update.outputs.branch }}
+          force: true
+
+      - name: Open a pull request
+        if: steps.update.outputs.pull_request == 'true'
+        run: hub pull-request --no-edit
+        env:
+          GITHUB_TOKEN: ${{ secrets.HOMEBREW_GITHUB_API_TOKEN }}