Bug: Git grep -f reads the filename relative to the repository root

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

In the Git repository, I ran

    echo tig > pattern-file &&
        echo git > xdiff/pattern-file &&
        cd xdfiff &&
        git grep -f pattern-file

What did you expect to happen? (Expected behavior)

Git grep -f to read the pattern-file, in the xdiff directory and search for lines matching `git` in the xdiff directory.

What happened instead? (Actual behavior)

Git grep -f reads the filename, relative to the Git root and searches for lines matching `tig` in the xdiff directory.

What's different between what you expected and what actually happened?

The file that Git grep uses for patterns is read relative to the root of the Git repository, and not the current directory.

Anything else you want to add:

Please review the rest of the bug report below. You can delete any lines you don't wish to share.

[System Info] git version: git version 2.42.0 cpu: x86_64 no commit associated with this build sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.15.90.1-microsoft-standard-WSL2 #1 SMP Fri Jan 27 02:56:13 UTC 2023 x86_64 compiler info: gnuc: 11.4 libc info: glibc: 2.35 $SHELL (typically, interactive shell): /bin/bash