Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Closes #53604.

When `log.showSignature = true`, git prepends signature verification
lines to stdout before the `--format` output. The null-separated parsers
in `crates/git/src/repository.rs` weren't expecting that, so SHAs ended
up corrupted — the git graph detail panel showed "0 changed files" for
every commit, and file history was similarly broken.

Setting `-c log.showSignature=false` in `build_command` is the same
trick we already use for `core.fsmonitor`. It only affects
log/show/whatchanged, so other commands aren't touched.

Verified locally with an SSH-signed repo: before this change the detail
panel said "0 changed files"; after, the modified files show up
correctly.

Release Notes:

- git_graph: Fix breakage that occurs when `log.showSignature` is
enabled

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Co-authored-by: Anthony Eid <anthony@zed.dev>