perf: avoid listing modified files twice in checkout-file

[cjappl]

_forgit_checkout_file called _forgit_list_modified_files once to check for an empty result and again to feed fzf, running git diff twice on every invocation. Cache the result and reuse it for both checks.

Check list

• I have performed a self-review of my code
• I have commented my code in hard-to-understand areas
• I have added unit tests for my code
• I have made corresponding changes to the documentation

Description

Type of change

• Performance

Test environment

• Shell
  • bash
  • zsh
  • fish
• OS
  • Linux
  • Mac OS X
  • Windows
  • Others:

Summary by CodeRabbit

• Bug Fixes
  • Improved the modified-file checkout picker by reusing a single, consistent list of modified files for both the empty-state check and the selection display.
  • Prevented cases where the candidates shown in the picker could differ from the availability logic used to decide whether files were selectable.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: dc16b0bc-c437-41d4-ba40-8d8b934b9c18

📥 Commits

Reviewing files that changed from the base of the PR and between 7890a06 and 88ddf4a.

📒 Files selected for processing (1)

• bin/git-forgit

🚧 Files skipped from review as they are similar to previous changes (1)

• bin/git-forgit

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📝 Walkthrough

Walkthrough

_forgit_checkout_file captures modified files once, uses that value for the empty-list check, and passes the same list to fzf for checkout target selection.

Changes

Checkout file selection

Layer / File(s)	Summary
Capture and reuse modified files bin/git-forgit	_forgit_checkout_file stores the modified-file output once, checks whether it is empty, and feeds the captured list to fzf.

Estimated code review effort: 2 (Simple) | ~5 minutes

Possibly related PRs

• wfxr/forgit#544: Both changes update the checkout-file flow to use _forgit_list_modified_files for the empty-list check and fzf candidates.

Suggested reviewers: wfxr, carlfriedrich

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is concise and accurately summarizes the main performance change.
Description check	✅ Passed	The description is mostly complete with a summary, checklist, and test environment, though the Description section is sparse.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch capple/dont_fetch_files_twice

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[cjappl]

This originated from a coderabbit review here:
#544 (review)

[sandr01d]

Looks good, just one comment.

[sandr01d]

LGTM