fix: handle file mode changes (old mode/new mode) in diff parsing

Files with `old mode`/`new mode` extended headers were not parsed
correctly, causing two bugs:

1. `new mode` lines were not recognized as extended headers. The
   ExtendedHeader constants included `Old: 'old'` which matches
   `old mode ...`, but only had `NewFile: 'new file'` for new-prefixed
   headers. Since `'new mode ...'` does not start with `'new file'`,
   `parseExtendedHeader` returned null, breaking the extended header
   loop prematurely.

2. When `parseFileChange` returned undefined (due to unrecognized
   headers and no matching return conditions), `parseFileChanges`
   used `break` instead of `continue`, causing ALL subsequent files
   in the diff to be silently dropped.

Fixes:
- Add `NewMode: 'new mode'` to ExtendedHeader constants
- Handle mode-only changes (no --- /+++ markers, no chunks) by
  falling back to the comparison line path
- Change `parseFileChanges` to skip unparseable entries instead of
  aborting the entire parse

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: jkoppel

src/__fixtures__/mode-change

@@ -0,0 +1,3 @@
+diff --git a/script.sh b/script.sh
+old mode 100644
+new mode 100755

src/__fixtures__/mode-change-followed-by-file

@@ -0,0 +1,11 @@
+diff --git a/script.sh b/script.sh
+old mode 100644
+new mode 100755
+diff --git a/readme.md b/readme.md
+index abc1234..def5678 100644
+--- a/readme.md
++++ b/readme.md
+@@ -1,3 +1,3 @@
+ # Title
+-old content
++new content

src/__fixtures__/mode-change-with-content

@@ -0,0 +1,10 @@
+diff --git a/script.sh b/script.sh
+old mode 100644
+new mode 100755
+index abc1234..def5678
+--- a/script.sh
++++ b/script.sh
+@@ -1,3 +1,3 @@
+ #!/bin/bash
+-echo "hello"
++echo "world"

src/__tests__/__snapshots__/mode-change.test.ts.snap

@@ -0,0 +1,107 @@
+// Jest Snapshot v1, https://jestjs.io/docs/snapshot-testing
+
+exports[`mode-change parse mode change followed by another file 1`] = `
+{
+  "files": [
+    {
+      "chunks": [],
+      "path": "script.sh",
+      "type": "ChangedFile",
+    },
+    {
+      "chunks": [
+        {
+          "changes": [
+            {
+              "content": "# Title",
+              "lineAfter": 1,
+              "lineBefore": 1,
+              "type": "UnchangedLine",
+            },
+            {
+              "content": "old content",
+              "lineBefore": 2,
+              "type": "DeletedLine",
+            },
+            {
+              "content": "new content",
+              "lineAfter": 2,
+              "type": "AddedLine",
+            },
+          ],
+          "context": undefined,
+          "fromFileRange": {
+            "lines": 3,
+            "start": 1,
+          },
+          "toFileRange": {
+            "lines": 3,
+            "start": 1,
+          },
+          "type": "Chunk",
+        },
+      ],
+      "path": "readme.md",
+      "type": "ChangedFile",
+    },
+  ],
+  "type": "GitDiff",
+}
+`;
+
+exports[`mode-change parse mode change with content changes 1`] = `
+{
+  "files": [
+    {
+      "chunks": [
+        {
+          "changes": [
+            {
+              "content": "#!/bin/bash",
+              "lineAfter": 1,
+              "lineBefore": 1,
+              "type": "UnchangedLine",
+            },
+            {
+              "content": "echo "hello"",
+              "lineBefore": 2,
+              "type": "DeletedLine",
+            },
+            {
+              "content": "echo "world"",
+              "lineAfter": 2,
+              "type": "AddedLine",
+            },
+          ],
+          "context": undefined,
+          "fromFileRange": {
+            "lines": 3,
+            "start": 1,
+          },
+          "toFileRange": {
+            "lines": 3,
+            "start": 1,
+          },
+          "type": "Chunk",
+        },
+      ],
+      "path": "script.sh",
+      "type": "ChangedFile",
+    },
+  ],
+  "type": "GitDiff",
+}
+`;
+
+exports[`mode-change parse mode-only change 1`] = `
+{
+  "files": [
+    {
+      "chunks": [],
+      "path": "script.sh",
+      "type": "ChangedFile",
+    },
+  ],
+  "type": "GitDiff",
+}
+`;

src/__tests__/mode-change.test.ts

@@ -0,0 +1,42 @@
+import { getFixture } from './test-utils';
+import parseGitDiff from '../parse-git-diff';
+
+describe('mode-change', () => {
+  it('parse mode-only change', () => {
+    const fixture = getFixture('mode-change');
+    const result = parseGitDiff(fixture);
+    expect(result.files).toHaveLength(1);
+    expect(result.files[0]).toMatchObject({
+      type: 'ChangedFile',
+      path: 'script.sh',
+    });
+    expect(result).toMatchSnapshot();
+  });
+
+  it('parse mode change with content changes', () => {
+    const fixture = getFixture('mode-change-with-content');
+    const result = parseGitDiff(fixture);
+    expect(result.files).toHaveLength(1);
+    expect(result.files[0]).toMatchObject({
+      type: 'ChangedFile',
+      path: 'script.sh',
+    });
+    expect(result.files[0].chunks).toHaveLength(1);
+    expect(result).toMatchSnapshot();
+  });
+
+  it('parse mode change followed by another file', () => {
+    const fixture = getFixture('mode-change-followed-by-file');
+    const result = parseGitDiff(fixture);
+    expect(result.files).toHaveLength(2);
+    expect(result.files[0]).toMatchObject({
+      type: 'ChangedFile',
+      path: 'script.sh',
+    });
+    expect(result.files[1]).toMatchObject({
+      type: 'ChangedFile',
+      path: 'readme.md',
+    });
+    expect(result).toMatchSnapshot();
+  });
+});

src/constants.ts

@@ -15,6 +15,7 @@ export const FileType = {
 export const ExtendedHeader = {
   Index: 'index',
   Old: 'old',
+  NewMode: 'new mode',
   Copy: 'copy',
   Similarity: 'similarity',
   Dissimilarity: 'dissimilarity',

src/parse-git-diff.ts

@@ -32,7 +32,8 @@ function parseFileChanges(ctx: Context): AnyFileChange[] {
   while (!ctx.isEof()) {
     const changed = parseFileChange(ctx);
     if (!changed) {
-      break;
+      ctx.nextLine();
+      continue;
     }
     changedFiles.push(changed);
   }
@@ -123,6 +124,12 @@ function parseFileChange(ctx: Context): AnyFileChange | undefined {
       chunks,
       path: chunks[0].pathAfter,
     };
+  } else if (comparisonLineParsed?.to) {
+    return {
+      type: FileType.Changed,
+      chunks,
+      path: comparisonLineParsed.to,
+    };
   }
   return;
 }