fix(git): git gc with safe.bareRepository=explicit

[weihanglo]

What does this PR try to resolve?

Git v3 will enforce safe.bareRepository=explicit,
while older versions don't.
See https://git-scm.com/docs/BreakingChanges/2.55.0#_git_3_0

This ensures git gc work with safe.bareRepository=explicit
by setting an explicit GIT_DIR environment variable.

This is not a requirement of making CLI fetch the default,
but better to have before Git CLI v3 is out.
CC #17329

How to test and review this PR?

You can basically repro this manually today with Git 2.38.0 and later (which safe.bareRepository first appears).

1. Set safe.bareRepository=explicit
2. cargo fetch with config net.git-fetch-with-cli=true
3. Put a marker file inside $CARGO_HOME/git/db/<git-dir>/
4. cargo update to force a git fetch
5. Observe the marker is gone, because Cargo reinitialized the entire db.

I am not sure if there is a better way to test it than a marker file though.

🤖 LLM disclosure: I told Codex to check if Cargo's git fetch usage is not compatibility with Git CLI v3, and surprisingly it told me git gc is not good. I didn't expect this.

[rustbot]

r? @epage

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: @epage, @weihanglo
• @epage, @weihanglo expanded to epage, weihanglo

[epage]

Always feel sketchy testing for indirect results because it makes the test brittle and kicks that can down the road to someone who feels responsible for making them still work.

[weihanglo]

Always feel sketchy testing for indirect results because it makes the test brittle and kicks that can down the road to someone who feels responsible for making them still work.

I am also fine without any test as it is pretty straightforward. How do you want to proceed?

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[epage]

Yeah, would be nice to have a tests generally but unsure if it is worth it here.

[weihanglo]

TIL jj push after jj bookmark delete deletes remote bookmarks. I should have read the manual first

[zzz6519003]

might add test as well? XD

sorry for duplication