add script for git bisect on a performance test

[tdhock]

Closes #7688

[github-actions]

No obvious timing issues in HEAD=bisect-atime

Generated via commit ce8cea2

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 16 seconds
Installing different package versions	24 seconds
Running and plotting the test cases	4 minutes and 25 seconds

[tdhock]

I run like this

$ bash bisect-7687.sh
cp: cannot stat 'tests.R': No such file or directory
Previous HEAD position was 9a8f6649 Bump codecov/codecov-action from 5 to 6 (#7683)
Switched to branch 'bisect-atime'
A	.ci/atime/bisect-7687.R
D	.ci/atime/tests.R
Your branch is up to date with 'origin/bisect-atime'.
status: waiting for both good and bad commits
status: waiting for bad commit, 1 good commit known
Bisecting: 100 revisions left to test after this (roughly 7 steps)
[d55a69a7e2c80173f4d8c2ad62fab06ac603c61e] Improve translation-readiness of string with subclause (#7476)
running 'bash' 'C:/Users/hoct2726/R/data.table/.ci/atime/run-7687.sh'
During startup - Warning message:
Setting LC_CTYPE=C.UTF-8 failed 
Warning message:
In atime::atime_pkg_test_info("~/R/data.table") :
  CRAN version=1.18.2.1 but installed version=1.18.99 fix via install.packages('data.table')
 [1] "fread(colClasses=list(Date='date')) improved in #6107"
 [2] "fread(colClasses='Date') improved in #6107"           
…
  [<-.IDate                data.table.20b463c9592c07d05ad2e89a25f84f0b998d6154
  edit.data.table          data.table.20b463c9592c07d05ad2e89a25f84f0b998d6154
Key: <N>
       N   Before     HEAD ratio
   <int>    <num>    <num> <num>
1: 10000 369.3516 369.3516     1
0216983c51e03e3f61d5e6f08f4ba0c42cceb22c is the first new commit
commit 0216983c51e03e3f61d5e6f08f4ba0c42cceb22c
Author: Benjamin Schwendinger <52290390+ben-schwen@users.noreply.github.com>
Date:   Thu Jan 15 10:47:12 2026 +0100

    modularize optimization internals (#7401)
    …
 NEWS.md                   |   8 +
 R/data.table.R            | 721 +++++++++++++++++++++-------------
 R/test.data.table.R       |  34 +-
 inst/tests/benchmark.Rraw |  26 +-
 inst/tests/optimize.Rraw  | 486 +++++++++++++++++++++++
 inst/tests/tests.Rraw     | 978 +++++++++-------------------------------------
 man/test.Rd               |   3 +-
 src/gsumm.c               |   2 +-
 tests/optimize.R          |   2 +
 9 files changed, 1171 insertions(+), 1089 deletions(-)
 create mode 100644 inst/tests/optimize.Rraw
 create mode 100644 tests/optimize.R
bisect found first bad commit

it finds the right commit, but the code is not very portable.