git grep --no-index fails when given an absolute path

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)
- Run `git grep --no-index -r -- -- /usr/share/fish/completions/`

What did you expect to happen? (Expected behavior)

The command executes the search successfully.

What happened instead? (Actual behavior)

This happens:

triallax@satoru ~> git grep --no-index -r -- -- /usr/share/fish/completions/
BUG: environment.c:213: git environment hasn't been setup
fish: Job 1, 'git grep --no-index -r -- -- /u…' terminated by signal SIGABRT (Abort)

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

One of them is an error, and the other is a successful execution.

Anything else you want to add:

Interestingly enough, this seems to be happening only with absolute paths from my testing, and it happens even when the directory that's passed is a Git repo.

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 6.5.8_1 #1 SMP PREEMPT_DYNAMIC Fri Oct 20 13:35:41 UTC 2023 x86_64 compiler info: gnuc: 12.2 libc info: glibc: 2.36 $SHELL (typically, interactive shell): /bin/dash

[Enabled Hooks] not run from a git repository - no hooks to show