Use treeless clone to fetch enough history for diff

A treeless clone contains all git history (and so can always find the
correct merge base), but is more efficient than a normal clone/fetch.

In order to use both treeless clone and fetch operations, we now manage
git clone, fetch and checkout operations ourselves rather than relying
on actions/checkout.

README.md

@@ -27,14 +27,11 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      # Check out a copy of this repository so that we can compute a diff to
-      # evaluate and load the Action's configuration.
-      - name: Checkout this repository
-        uses: actions/checkout@v4
-
-      # Run the estimation tool
       - name: Run sizeup
         # TODO: Replace the version below with your desired version.
+        #
+        # For more details please see:
+        # https://github.com/lerebear/sizeup-action/blob/main/README.md#versioning
         uses: lerebear/sizeup-action@v0.4.2
         id: sizeup-action
 
@@ -49,23 +46,29 @@ jobs:
           # This input is required.
           token: "${{ secrets.GITHUB_TOKEN }}"
 
-          # Optional arguments that will be forwarded to `git diff` when
-          # computing the diff to evaluate with this Action.
+          # Options that will be forwarded to `git diff` when computing the
+          # diff to evaluate with this workflow.
           #
           # Defaults to "--ignore-space-change", which ignores lines of the
           # diff in which the only change is to the amount of whitespace on the
           # line.
-          git-diff-args: ""
+          git-diff-options: ""
 
-          # Path to a YAML configuration file for this Action that is stored in
-          # this repository.
+          # Path to a YAML configuration file for this workflow that is stored
+          # in this repository.
           #
-          # This input defaults to "", which instructs the Action to use the
+          # This input defaults to "", which instructs the workflow to use the
           # built-in default configuration.
           configuration-file-path: ".github/workflows/sizeup/config.yaml"
 ```
 
-This will use `sizeup` to estimate the reviewability of any pull request opened on your repository using a YAML configuration file found at `.github/workflows/sizeup/config.yaml`. The format of the configuration file is described below.
+This will use [`sizeup`](https://github.com/lerebear/sizeup-core) to estimate the reviewability of any pull request opened on your repository using a YAML configuration file found at `.github/workflows/sizeup/config.yaml`. The format of the configuration file is described below.
+
+Please note that the workflow configuration above does not use [`actions/checkout`](https://github.com/actions/checkout). This is because `actions/checkout` does not provide enough options to make a customized `git diff` command maximally efficient. Instead, this Action will perform its own clone, fetch, and checkout operations using the provided `token`.
+
+## Versioning
+
+This Action follows [semantic versioning conventions](https://semver.org), but is still <1.0. Thus, as is common for pre-1.0 software, breaking changes are  sometimes introduced in minor version bumps (although patch version bumps will only contain backwards-compatible bug fixes). Please bear this in mind when choosing the version of this Action that you would like to use.
 
 ## Configuration
 

__tests__/main.test.ts

@@ -9,21 +9,31 @@
 import * as core from '@actions/core'
 import * as main from '../src/main'
 import * as initializer from '../src/initializer'
+import { Git } from '../src/git'
 import * as github from '@actions/github'
 
 function pullRequestEventContext(overrides = {}): object {
   return {
     eventName: 'pull_request',
+    repo: {
+      owner: 'lerebear',
+      name: 'sizeup-action'
+    },
     payload: {
       pull_request: {
         base: {
+          ref: 'main',
           repo: {
+            full_name: 'lerebear/sizeup-action',
             name: 'sizeup-action',
             owner: {
               login: 'lerebear'
             }
           }
         },
+        head: {
+          ref: 'topic'
+        },
         labels: [],
         number: 1,
         user: {
@@ -69,9 +79,12 @@ describe('action', () => {
   // Shallow clone original @actions/github context
   const originalContext = { ...github.context }
 
+  // Mock cloning the repo
+  jest.spyOn(Git.prototype, 'clone').mockImplementation(async () => {})
+
   // Mock the diff that we use for evaluation.
   jest
-    .spyOn(initializer, 'fetchDiff')
+    .spyOn(Git.prototype, 'diff')
     .mockImplementation(async () =>
       Promise.resolve(
         '--- README.md	2023-10-16 16:35:38\n+++ README-AGAIN.md	2023-10-16 16:36:07\n@@ -0,0 +1 @@\n+# Hello, World!'

action.yml

@@ -6,9 +6,9 @@ inputs:
   token:
     description: 'GitHub API token with read permissions for pull requests in this repository.'
     required: true
-  git-diff-args:
+  git-diff-options:
     description: |
-      Optional custom arguments to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
+      Custom options to forward to `git diff` when retrieving the diff of the pull request that triggered this workflow.
       The result of that `git diff` command will be used for evaluation with sizeup.
     default: '--ignore-space-change'
   configuration-file-path: