move git commit checker to separate repository

The YAML and Python script originated from the OMPI repository (diverged here:
open-mpi/ompi@ef866a4)

An OMPI maintainer wanted this and the label bot in two of their other repos.
Instead of copying the code to their repo(s), it is far simpler to move the
actions to separate repos and publish them on the GitHub marketplace.

The functionality remains the same, but the YAML had to be tweaked to work as a
reusable (marketplace) action. These changes are:

* add / change the action name, description, branding, and inputs
* remove the trigger
* remove permissions
* specify composite action (runs:)
* remove os specification
* specify shell for each command run
* use github.action_path instead of the env variable GITHUB_WORKSPACE

Instead of using JSON for config, we instead take the values as inputs, and give
them the same defaults in the JSON/code. This resulted in a minor change to the
Python script: removing the loading of the JSON file and instead get values from
the environment.

Signed-off-by: Joe Downs <[email protected]>

Author: Joe-Downs

README.md

@@ -0,0 +1,13 @@
+# Pull Request Git Commit Checker
+A minor action which checks commits on a pull request for certain conformity
+policies. If all commits pass, the action succeeds and no comment emitted. If a
+check fails for one or many commits, the action fails and a comment is placed on
+the PR explaining which commits caused the failure and why. For instance, if
+contributed commits need to be signed off, a comment with this message will
+appear:
+
+`e3042c1`**: foobar**
+* *check_signed_off: does not contain a valid Signed-off-by line*
+
+
+*Uses [SemVer 2.0.0](https://semver.org/spec/v2.0.0.html)*

action.yaml

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+name: Pull Request Git Commit Checker
+description: Checks pull request commits for policy conformity
+branding:
+  icon: check-square
+  color: green
+inputs:
+  token:
+    description: "The GITHUB_TOKEN secret"
+    required: true
+  cherry-pick-required:
+    description: "Whether the cherry-pick message is required"
+    required: false
+    default: false
+  permit-empty:
+    description: "Allow empty commit messages"
+    required: false
+    default: false
+
+runs:
+  using: "composite"
+  steps:
+  - name: Check out the code
+    uses: actions/checkout@v2
+    with:
+      # Get all branches and history
+      fetch-depth: 0
+
+  - name: Setup Python
+    uses: actions/setup-python@v2
+    with:
+      python-version: '3.x'
+
+  - name: Get the GitPython and PyGithub modules
+    run: pip install gitpython PyGithub
+    shell: bash
+
+  - name: Check all git commits
+    run: ${{ github.action_path }}/git-commit-checks.py
+    shell: bash
+    env:
+      GITHUB_TOKEN: ${{ inputs.token }}
+      CHERRY_PICK_REQUIRED: ${{ inputs.cherry-pick-required }}
+      PERMIT_EMPTY: ${{ inputs.permit-empty }}
+      PR_NUM: ${{ github.event.number }}