fix: handle identical old/new content

[elwynelwyn]

Fix for #16

When creating a diff file where old / new content is identical, currently there is a crash - because comparing the same content will have no hunks, and code always tries to create hunks.

This PR changes diff-parse from a do/while loop, to a while loop, to ensure peek() is called initially before trying to create hunks.

For the case content is identical, empty hunks array will be created instead of Expected hunk header but reached end of diff exception.

Test case to reproduce the bug / demonstrate the fix

#!/usr/bin/env node

// build package first
import { generateDiffFile } from "./packages/file/index.js";

const identicalContent = `hello world
line 2
line 3`;

try {
  const diffFile = generateDiffFile(
    "test.txt",
    identicalContent,
    "test.txt",
    identicalContent,
    "txt",
    "txt"
  );

  diffFile.init();

  diffFile.buildSplitDiffLines();

  diffFile.buildUnifiedDiffLines();

  console.log(`diff file built:
- Additions: ${diffFile.additionLength}
- Deletions: ${diffFile.deletionLength}
- Split lines: ${diffFile.splitLineLength}
- Unified lines: ${diffFile.unifiedLineLength}`);
} catch (error) {
  console.error(`oh no: ${error.message}`, error);
}