Create the git.emptyTreeCommit() method to get the commit hash of the repo in its initial state prior to the first commit being applied.

More future-proof than relying on a hard-coded value for the hash should `git` start using the SHA256 value in place of SHA1.

Author: steveukx

.changeset/blank-club-mauve.md

@@ -0,0 +1,4 @@
+---
+'simple-git': minor
+---
+Create new interface `git.emptyTreeCommit` to obtain the commit hash for the repo prior to the first commit.

simple-git/readme.md

@@ -285,10 +285,11 @@ in v2 (deprecation notices were logged to `stdout` as `console.warn` in v2).
 -  `.grep(searchTerm)` searches for a single search term across all files in the working tree, optionally passing a standard [options](#how-to-specify-options) object of additional arguments
 -  `.grep(grepQueryBuilder(...))` use the `grepQueryBuilder` to create a complex query to search for, optionally passing a standard [options](#how-to-specify-options) object of additional arguments
 
-## git hash-object
+## git hash-object / hash properties
 
 -  `.hashObject(filePath, write = false)` computes the object ID value for the contents of the named file (which can be
    outside of the work tree), optionally writing the resulting value to the object database.
+- `.emptyTreeCommit()` gets the commit hash for the repo in its initial empty state before the `git.firstCommit` was applied, useful for computing the insert-only initial commit with `git.diffSummary`.   
 
 ## git init
 

simple-git/src/lib/simple-git-api.ts

@@ -5,6 +5,7 @@ import checkout from './tasks/checkout';
 import countObjects from './tasks/count-objects';
 import commit from './tasks/commit';
 import config from './tasks/config';
+import emptyTreeCommit from './tasks/empty-tree-commit';
 import firstCommit from './tasks/first-commit';
 import grep from './tasks/grep';
 import { hashObjectTask } from './tasks/hash-object';
@@ -147,6 +148,7 @@ Object.assign(
    commit(),
    config(),
    countObjects(),
+   emptyTreeCommit(),
    firstCommit(),
    grep(),
    log(),

simple-git/src/lib/tasks/empty-tree-commit.ts

@@ -0,0 +1,15 @@
+import { Response, SimpleGit } from '../../../typings';
+import { SimpleGitApi } from '../simple-git-api';
+import { trailingFunctionArgument } from '../utils';
+import { straightThroughStringTask } from './task';
+
+export default function (): Pick<SimpleGit, 'emptyTreeCommit'> {
+   return {
+      emptyTreeCommit(this: SimpleGitApi): Response<string> {
+         return this._runTask(
+            straightThroughStringTask(['hash-object', '-t', 'tree', '/dev/null'], true),
+            trailingFunctionArgument(arguments)
+         );
+      },
+   };
+}

simple-git/test/integration/empty-tree-commit.spec.ts

@@ -0,0 +1,13 @@
+import { createTestContext } from '@simple-git/test-utils';
+
+const EMPTY_SHA = '4b825dc642cb6eb9a060e54bf8d69288fbee4904';
+const EMPTY_SHA_256 = '6ef19b41225c5369f1c104d45d8d85efa9b057b53b14b4b9b939dd74decc5321';
+
+describe('empty-tree-commit', () => {
+   it('gets the empty tree commit', async () => {
+      const context = await createTestContext();
+      const commit = await context.git.emptyTreeCommit();
+
+      expect(commit === EMPTY_SHA || commit === EMPTY_SHA_256).toBe(true);
+   });
+});

simple-git/typings/simple-git.d.ts

@@ -524,6 +524,11 @@ export interface SimpleGit extends SimpleGitBase {
 
    diffSummary(callback?: types.SimpleGitTaskCallback<resp.DiffResult>): Response<resp.DiffResult>;
 
+   /**
+    * Gets the commit hash of the initial empty repo commit, immediately prior to the first commit in the repo
+    */
+   emptyTreeCommit(callback?: types.SimpleGitTaskCallback<string>): Response<string>;
+
    /**
     * Sets an environment variable for the spawned child process, either supply both a name and value as strings or
     * a single object to entirely replace the current environment variables.