fix: resolve --file paths from subdirectories

[lawrence3699]

Fixes #2871

Before this change, --file only resolved reliably when GitUI was started from the repository root. From a subdirectory, a cwd-relative path like src/index.html normalized to ./src/index.html instead of the repo-relative ./frontend/src/index.html, and a repo-root-relative path like frontend/src/index.html failed to resolve at all.

This keeps the existing repo-relative selection flow, but resolves relative paths against both the current directory and the repository root before converting them back to a repo-relative path. That makes --file work from subdirectories for both path styles without changing the rest of the Files tab behavior.

Validation:

• cargo test 'app::tests::resolve_initial_select_file_from_repo_root' -- --exact\n- cargo test 'app::tests::resolve_initial_select_file_from_subdir_cwd_relative_path' -- --exact\n- cargo test 'app::tests::resolve_initial_select_file_from_subdir_repo_relative_path' -- --exact\n- cargo test 'gitui::tests::gitui_starts' -- --exact\n- cargo fmt --check\n- cargo clippy --bin gitui -- -D warnings

[Copilot]

Pull request overview

Fixes --file path resolution when GitUI is launched from a subdirectory by resolving relative paths against both the current working directory and the repository root, then converting the result back into a repo-relative path for initial selection.

Changes:

• Replace inline --file canonicalization/strip-prefix logic with a dedicated resolve_initial_select_file* helper.
• Resolve relative --file inputs against both cwd and repo root before selecting the file.
• Add unit tests covering repo-root invocation and subdirectory invocation with both cwd-relative and repo-relative inputs.

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[Copilot]

The new unit tests spawn the git binary (Command::new("git")) to initialize a repo. If the project tries to keep unit tests runnable without requiring an external git executable on PATH, consider using the existing git2-testing helper (used in src/gitui.rs tests) or git2::Repository::init instead.

Suggested change

	use std::{fs, path::Path, process::Command};
	use tempfile::TempDir;
	fn make_repo_with_nested_file() -> (TempDir, RepoPath) {
	let temp_dir = TempDir::new().expect("create temp repo");
	let status = Command::new("git")
	.arg("init")
	.arg("-q")
	.arg(temp_dir.path())
	.status()
	.expect("run git init");
	assert!(status.success(), "git init should succeed");
	use std::{fs, path::Path};
	use tempfile::TempDir;
	fn make_repo_with_nested_file() -> (TempDir, RepoPath) {
	let temp_dir = TempDir::new().expect("create temp repo");
	git2::Repository::init(temp_dir.path())
	.expect("initialize git repository");