When a repository contains different kinds of files, it may be desirable to
use different algorithms based on file type. This is currently not feasible
through the command line or using git configs. However, we can leverage the
fact that gitattributes are path aware.

Teach the diff machinery to check gitattributes when diffing files by using
the existing diff. scheme, and add an "algorithm" type to the external
driver config.

Changes since V2:

 * minor clean up and variable renaming
 * avoid parsing attribute files for the driver if the diff algorithm is set
   through the command line

Changes since V1:

 * utilize the existing diff.<driver>.* scheme where the driver is defined
   in gitattributes, but the algorithm is defined in the gitconfig.

To address some of the performance concerns in the previous series, a
benchmark shows that a performance penalty is no longer incurred, now that
we are no longer adding an additional attributes parsing call:

$ hyperfine -r 5 -L a bin-wrappers/git,git '{a} diff v2.0.0 v2.28.0'
Benchmark 1: git-bin-wrapper diff v2.0.0 v2.28.0 Time (mean ± σ): 1.072 s ±
0.289 s [User: 0.626 s, System: 0.081 s] Range (min … max): 0.772 s … 1.537
s 5 runs

Benchmark 2: git diff v2.0.0 v2.28.0 Time (mean ± σ): 1.003 s ± 0.065 s
[User: 0.684 s, System: 0.067 s] Range (min … max): 0.914 s … 1.091 s 5 runs

Summary 'git diff v2.0.0 v2.28.0' ran 1.07 ± 0.30 times faster than
'git-bin-wrapper diff v2.0.0 v2.28.0'

John Cai (2):
  diff: consolidate diff algorithm option parsing
  diff: teach diff to read algorithm from diff driver

 Documentation/gitattributes.txt | 37 ++++++++++++++
 diff.c                          | 90 ++++++++++++++++++++++++---------
 diff.h                          |  1 +
 t/lib-diff-alternative.sh       | 38 +++++++++++++-
 userdiff.c                      |  4 +-
 userdiff.h                      |  1 +
 6 files changed, 146 insertions(+), 25 deletions(-)

base-commit: c867e4fa180bec4750e9b54eb10f459030dbebfd

Submitted-As: https://lore.kernel.org/git/pull.1452.v3.git.git.1676665285.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1452.git.git.1675568781.gitgitgadget@gmail.com
In-Reply-To: https://lore.kernel.org/git/pull.1452.v2.git.git.1676410819.gitgitgadget@gmail.com