[PATCH v7 0/7] speed up index load through parallelization

Thanks for all the feedback.

The biggest change since the last version is how this patch series interacts
with the split-index feature.  With a split index, most of the cache entries
are stored in the shared index so would benefit from multi-threaded parsing.
To enable that, the EOIE and IEOT extensions are now written into the shared
index (rather than being stripped out like the other extensions).

Because of this, I can now update the tests in t1700-split-index.sh to have
updated SHA values that include the EOIE extension instead of disabling the
extension.

Using p0002-read-cache.sh to generate some performance numbers shows how
each of the various patches contribute to the overall performance win on a
particularly large repo.

Repo w/3M files      Baseline  Optimize V4   Extensions      Entries
--------------------------------------------------------------------------
0002.1: read_cache   693.29    655.65 -5.4%  470.71 -32.1%   399.62 -42.4%

Note how this cuts nearly 300ms off the index load time!

Base Ref: master
Web-Diff: https://github.com/benpeart/git/commit/c1125a5d9a
Checkout: git fetch https://github.com/benpeart/git read-index-multithread-v7 && git checkout c1125a5d9a


### Patches

Ben Peart (6):
  read-cache: clean up casting and byte decoding
  eoie: add End of Index Entry (EOIE) extension
  config: add new index.threads config setting
  read-cache: load cache extensions on a worker thread
  ieot: add Index Entry Offset Table (IEOT) extension
  read-cache: load cache entries on worker threads

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

 Documentation/config.txt                 |   7 +
 Documentation/technical/index-format.txt |  41 ++
 config.c                                 |  18 +
 config.h                                 |   1 +
 read-cache.c                             | 749 +++++++++++++++++++----
 t/README                                 |   5 +
 t/t1700-split-index.sh                   |  13 +-
 7 files changed, 715 insertions(+), 119 deletions(-)


base-commit: fe8321ec057f9231c26c29b364721568e58040f7
-- 
2.18.0.windows.1