Add git read-tree completions

[rasitakyol]

Summary

• Add completions for git read-tree.
• Cover the documented read-tree flags, including prefix/index-output and submodule controls.
• Reuse the existing branch, remote branch, ref, and tag generators for the optional variadic tree-ish argument.

Linked issues

Closes #297

Upstream request: warpdotdev/warp#12321

Testing

• ./script/presubmit
• cargo test -p warp-command-signatures --lib (66/66)
• git diff --check
• Manually verified git read-tree <Tab> in a local Warp build using this checkout.

Screenshot

[oz-for-oss]

@rasitakyol

Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

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[oz-for-oss]

@rasitakyol

Every PR must be linked to a same-repo issue before Oz can review it.

Next step: open or find a same-repo issue describing this change, then link it to this PR by adding Closes #123 to the PR description (or using the "Development" sidebar on GitHub). A maintainer will mark the issue ready-to-implement when it is ready. Once it is marked, comment /oz-review to re-trigger review.

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[rasitakyol]

Addressed the same-repository issue requirement: I created #297 and updated this PR to link it with Closes #297, while retaining warpdotdev/warp#12321 as the upstream request.

Issue #297 is now undergoing Oz triage and awaiting the maintainer-applied ready-to-implement label. I will re-trigger /oz-review once that readiness gate is satisfied.