feat: Initial commit

Dockerfile

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+FROM exions/github-actions-git:latest
+
+COPY entrypoint.sh /entrypoint.sh
+
+RUN chmod u+x /entrypoint.sh
+
+ENTRYPOINT ["/entrypoint.sh"]

LICENSE

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+MIT License
+
+Copyright (c) 2020 eXions
+
+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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+# Merge Upstream
+Merge changes from an upstream repository branch into a current repository branch. For example, updating changes from the repository that was forked from.
+
+Current limitations:
+- only merge only selected branch
+- branch has to be of the same name
+- only work with public upstream Github repository
+
+## Usage
+
+### Set up for manual trigger
+copy and commit this to `.github/workflows/merge-upstream.yml` in your default branch of your repository.
+
+```yaml
+name: Manual Merge Remote Action
+on: 
+  workflow_dispatch:
+    inputs:
+      upstream:
+        description: 'Upstream repository owner/name. Eg. exions/merge-upstream'
+        required: true
+        default: 'master'
+      branch:
+        description: 'Branch to merge'
+        required: true
+        default: 'master'
+
+jobs:
+  merge-upstream:
+    runs-on: ubuntu-latest
+    steps: 
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.inputs.branch }}
+          fetch-depth: 0 
+      - name: Merge Upstream
+        uses: exions/merge-upstream@v1
+        with:
+          upstream: ${{ github.event.inputs.upstream }}
+          branch: ${{ github.event.inputs.branch }}
+```
+
+### Set up for scheduled trigger
+
+```yaml
+name: Scheduled Merge Remote Action
+on: 
+  schedule:
+    - cron: '0 0 * * 1'
+    # scheduled for 00:00 every Monday
+
+jobs:
+  merge-upstream:
+    runs-on: ubuntu-latest
+    steps: 
+      - name: Checkout
+        uses: actions/checkout@v2
+        with:
+          ref: ${{ github.event.inputs.branch }}
+          fetch-depth: 0 
+      - name: Merge Upstream
+        uses: exions/merge-upstream@v1
+        with:
+          upstream: owner/repo    # set the upstream repo
+          branch: master          # set the branch to merge
+```
+
+Reference: 
+- [Triggering a workflow with events](https://docs.github.com/en/actions/configuring-and-managing-workflows/configuring-a-workflow#triggering-a-workflow-with-events)
+
+## How to run this action manually
+
+This action can trigger manually as needed. 
+
+1. Go to `Actions` at the top of your Github repository
+2. Click on `Manual Undo Push Action` (or other name you have given) under `All workflows`
+3. You will see `Run workflow`, click on it
+4. Fill in the branch to undo the most recent push (⚠️ make sure it is correct)
+5. Click `Run workflow`
+6. Check your branch commit history

action.yml

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+name: "Merge Upstream"
+description: "Merge changes from the uppstream repo you had forked from."
+author: "eXions"
+branding:
+  icon: git-merge
+  color: black
+
+inputs:
+  upstream:
+    description: 'Upstream repository owner/name. For example, exions/merge-upstream'
+    required: true
+  branch:
+    description: 'This branch to merge from and to. For example, master'
+    default: 'master'
+
+runs:
+  using: 'docker'
+  image: 'Dockerfile'
+  args:
+    - ${{ inputs.upstream }}
+    - ${{ inputs.branch }}

entrypoint.sh

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+#!/bin/sh -eux
+
+cd "${GITHUB_WORKSPACE}"
+
+git remote add -f  "https://github.com/$1.git"
+git branch ${GITHUB_REF}
+git merge --ff-only upstream/${GITHUB_REF}
+git push