unpack_strategy/directory: use mv for nested unpack

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

mv should preserve hardlinks and allow faster unpack on the same filesystem. A secondary pass is done with cp to copy over attributes onto any existing directories.

We only run this for nested unpacks as most direct Directory strategy usage is for repositories where moving files breaks existing code.

This uses cp -pR for non-move as some potential user reported issues could be due to Apple's cp -l on specific macOS versions. Can consider re-adding cp -l with better handling for older macOS.

---

This improves performance when pouring bottles if user has brew installed on same filesystem as /tmp directory (~1.2-1.4x when tested via brew reinstall which means direct brew install).

---

macOS mv man page says:

     As the rename(2) call does not work across file systems, mv uses cp(1) and rm(1) to accomplish the move.  The
     effect is equivalent to:

           rm -f destination_path && \
           cp -pRP source_file destination && \
           rm -rf source_file

Which should essentially be similar to cp -pR when combined with other logic.

[cho-m]

Will need to think about this a bit more in relation to how we are "unpacking" git repositories.

[MikeMcQuaid]

Worth a try once 🟢 and you're ready, thanks!

[MikeMcQuaid]

Thanks @cho-m! Good to merge when you are.

[cho-m]

Will merge now and keep an eye out for any user issues.

Should speed up pour time for large bottle, e.g. for brew install texlive --display-times (2GB bottle, 4GB unpack), I see

• Old: 131.294 s
• New: 48.367 s

I did try removing nested part of extraction (directly extracting to cellar), which ran in 46.952 s, but not sure on any edge cases. Not significant enough to continue exploring.