[PATCH v4 0/5] read-cache: speed up index load through parallelization

This version of the patch merges in Duy's work to speed up index v4 decoding. I had to massage it a bit to get it to work with the multi-threading but its still largely his code. It helps a little (3%-4%) when the cache entry thread(s) take the longest and not when the index extensions loading is the long thread.

I also added a minor cleanup patch to minimize the casting required when working with the memory mapped index and other minor changes based on the feedback received.

Base Ref: v2.19.0
Web-Diff: https://github.com/benpeart/git/commit/9d31d5fb20
Checkout: git fetch https://github.com/benpeart/git read-index-multithread-v4 && git checkout 9d31d5fb20

### Patches

Ben Peart (4):
  eoie: add End of Index Entry (EOIE) extension
  read-cache: load cache extensions on a worker thread
  read-cache: speed up index load through parallelization
  read-cache: clean up casting and byte decoding

Nguyễn Thái Ngọc Duy (1):
  read-cache.c: optimize reading index format v4

 Documentation/config.txt                 |   6 +
 Documentation/technical/index-format.txt |  23 +
 config.c                                 |  18 +
 config.h                                 |   1 +
 read-cache.c                             | 581 +++++++++++++++++++----
 5 files changed, 531 insertions(+), 98 deletions(-)

base-commit: 1d4361b0f344188ab5eec6dcea01f61a3a3a1670