Credit to alexmv who made this commit back in Dec, 2017 when he was at dbx.
I've rebased it and am submitting it now.

With fsmonitor enabled, git diff currently lstats every file in the repo
This makes use of the fsmonitor extension to skip lstat() calls on files
that fsmonitor judged as unmodified.

I was able to do some testing with/without this change in a large in-house
repo (~ 400k files)

-----------------------------------------
(1) With fsmonitor enabled - on master of git (2.29.0)
-----------------------------------------
../git/bin-wrappers/git checkout HEAD~200
strace -c ../git/bin-wrappers/git diff

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.64    4.358994          10    446257         3 lstat
  0.12    0.005353           7       764       360 open

(A subsequent call)
strace -c ../git/bin-wrappers/git diff

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 99.84    4.380955          10    444904         3 lstat
  0.06    0.002564         135        19           munmap
...

-----------------------------------------
(2) With fsmonitor enabled - with my patch
-----------------------------------------
../git/bin-wrappers/git checkout HEAD~200
strace -c ../git/bin-wrappers/git diff

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 50.72    0.003090         163        19           munmap
 19.63    0.001196         598         2           futex
...
  0.00    0.000000           0         4         3 lstat

-----------------------------------------
(3) With fsmonitor disabled entirely
-----------------------------------------

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 98.52    0.277085       92362         3           futex
  0.27    0.000752           4       191        63 open
...
  0.14    0.000397           3       158         3 lstat

I encoded this into a perf test with results as follow:

On master (2.29)

Test                                                             this tree
--------------------------------------------------------------------------------
7519.2: status (fsmonitor=.git/hooks/fsmonitor-watchman)         2.52(1.59+1.56)
7519.3: status -uno (fsmonitor=.git/hooks/fsmonitor-watchman)    0.18(0.12+0.06)
7519.4: status -uall (fsmonitor=.git/hooks/fsmonitor-watchman)   1.36(0.73+0.62)
7519.5: diff (fsmonitor=.git/hooks/fsmonitor-watchman)           0.85(0.30+0.54)
7519.7: status (fsmonitor=)                                      0.69(0.52+0.90)
7519.8: status -uno (fsmonitor=)                                 0.37(0.28+0.81)
7519.9: status -uall (fsmonitor=)                                1.53(0.93+1.32)
7519.10: diff (fsmonitor=)                                       0.34(0.26+0.81)

With this patch

Test                                                             this tree
--------------------------------------------------------------------------------
7519.2: status (fsmonitor=.git/hooks/fsmonitor-watchman)         2.84(1.70+1.76)
7519.3: status -uno (fsmonitor=.git/hooks/fsmonitor-watchman)    0.18(0.13+0.05)
7519.4: status -uall (fsmonitor=.git/hooks/fsmonitor-watchman)   1.35(0.81+0.53)
7519.5: diff (fsmonitor=.git/hooks/fsmonitor-watchman)           0.15(0.11+0.05)
7519.7: status (fsmonitor=)                                      0.71(0.54+0.90)
7519.8: status -uno (fsmonitor=)                                 0.38(0.30+0.81)
7519.9: status -uall (fsmonitor=)                                1.55(0.93+1.34)
7519.10: diff (fsmonitor=)                                       0.35(0.32+0.76)

Alex Vandiver (1):
  fsmonitor: use fsmonitor data in `git diff`

Nipunn Koorapati (3):
  t/perf/README: elaborate on output format
  t/perf/p7519-fsmonitor.sh: warm cache on first git status
  t/perf: add fsmonitor perf test for git diff

 diff-lib.c                | 17 +++++++++++++++--
 t/perf/README             |  2 ++
 t/perf/p7519-fsmonitor.sh | 19 ++++++++++++++++++-
 3 files changed, 35 insertions(+), 3 deletions(-)

base-commit: d4a392452e292ff924e79ec8458611c0f679d6d4

Submitted-As: https://lore.kernel.org/git/pull.756.git.1602968677.gitgitgadget@gmail.com