Ignore git warnings when diffing patch folders

packages/plugin-patch/sources/patchUtils.ts

@@ -299,8 +299,15 @@ export async function diffFolders(folderA: PortablePath, folderB: PortablePath)
 
   // we cannot rely on exit code, because --no-index implies --exit-code
   // i.e. git diff will exit with 1 if there were differences
-  if (stderr.length > 0)
-    throw new Error(`Unable to diff directories. Make sure you have a recent version of 'git' available in PATH.\nThe following error was reported by 'git':\n${stderr}`);
+  // git also writes warnings to stderr on success (empty HOME can make it
+  // complain about /.config/git/attributes), so ignore those lines.
+  const gitErrors = stderr
+    .split(/\r?\n/)
+    .filter(line => line.length > 0 && !/^warning:/i.test(line))
+    .join(`\n`);
+
+  if (gitErrors.length > 0)
+    throw new Error(`Unable to diff directories. Make sure you have a recent version of 'git' available in PATH.\nThe following error was reported by 'git':\n${gitErrors}`);
 
 
   const normalizePath = folderAN.startsWith(`/`)

packages/plugin-patch/tests/diffFolders.test.ts

@@ -1,3 +1,4 @@
+import {execUtils}      from '@yarnpkg/core';
 import {npath, NodeFS}  from '@yarnpkg/fslib';
 
 import {diffFolders}    from '../sources/patchUtils';
@@ -18,4 +19,22 @@ describe(`diffFolders`,  () => {
       expect(parsePatchFile(diff)).toMatchSnapshot();
     });
   }
+
+  it(`ignores git warnings on stderr`, async () => {
+    const spy = jest.spyOn(execUtils, `execvp`).mockResolvedValue({
+      code: 1,
+      stdout: `diff --git a/file.txt b/file.txt\n`,
+      stderr: `warning: unable to access '/.config/git/attributes': Permission denied\n`,
+    } as any);
+
+    try {
+      const diff = await diffFolders(
+        npath.toPortablePath(npath.join(fixtures, `update`, `a`)),
+        npath.toPortablePath(npath.join(fixtures, `update`, `b`)),
+      );
+      expect(diff).toContain(`diff --git`);
+    } finally {
+      spy.mockRestore();
+    }
+  });
 });