add support for "git bundle"

Please support https://git-scm.com/docs/git-bundle
Thankfully, you already have all the hard parts implemented!

Start by reading the linked git-bundle documentation and locating the existing isomorphic-git support for the related Git operations. No file, test, or entry point is named, so first identify where bundle handling belongs and which existing implementations are the stated hard parts. Done means git bundle is supported in the relevant environments with coverage for its behavior.

Written by the indexing model from the issue text.