R1:4b39fbe1155e

Allow modern hunks to be stored deflated

Summary: Ref T4045. Ref T5179. When saving a modern hunk, deflate it if we have the function and deflating it will save a nontrivial number of bytes.

Test Plan:
- Used `bin/hunks migrate` to move some hunks over, saw ~70-80% compression on most standard hunks.
- Viewed changesets using compressed hunks.
- Profiled `gzinflate()` and verified the cost is trivial (<< 1ms) at least for normal diffs.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers…