Expose basePath in WorkTree.prototype.entries

[as-cii]

Previously, when a rename occurred, it was impossible to determine the original path of a file.

With this pull request, when calling WorkTree.prototype.entries, each entry will now contain a basePath field that can either be:

• A null value indicating that the file didn't exist in the base commit.
• A Path representing the original location where the current Entry was at in the base commit.

This will be useful when an external index that exposes a diff-based interface needs to be kept up-to-date, e.g.:

if (entry.basePath) {
  if (entry.visible) {
    index.rename(entry.basePath, entry.path);
  } else {
    index.remove(entry.basePath);
  }
}