Converts large Mercurial repositories to Git/LFS significantly faster by integrating
the LFS conversion into the history export process.
Currently, converting large repositories requires two sequential, long-running steps:
1. Full history conversion (`hg` to `git`).
2. Full history rewrite/import (`git lfs import`).
For huge monorepos (100GiB+, 1M+ files), this sequence can take hours or days.
This commit introduces a new plugin that allows the repository to be converted *incrementally*
(JIT: Just-In-Time). The plugin identifies large files during the initial `hg` to `git`
conversion and immediately writes LFS pointers, eliminating the need for the second,
time-consuming history rewrite step.
