install() overwrites existing hooks despite README claiming idempotent behavior

[jgeluk]

Problem

The README states:

The install() call is idempotent. It won't modify the existing setup and hooks.

However, the implementation in src/lib.rs uses fs::write() directly without checking if files already exist:

fs::write(
    self._hooks_abs_dir(repo_path.as_str()).join("pre-commit"),
    r#"#!/bin/sh
. "$(dirname "$0")/_/sloughi.sh"

echo "Running Sloughi pre-commit..."
"#,
)?;

fs::write() truncates and overwrites existing files, meaning any custom pre-commit hook content is lost on every cargo build.

Expected Behavior

If a hook file already exists, install() should either:

1. Skip writing to that file entirely (true idempotent behavior)
2. Only update the _/sloughi.sh helper while preserving user hooks
3. Provide an option to force overwrite vs preserve existing

Reproduction

1. Add sloughi to build-dependencies with .custom_path(".githooks")
2. Create a custom .githooks/pre-commit with additional logic (fmt, clippy, etc.)
3. Run cargo build
4. Observe that .githooks/pre-commit is overwritten with the minimal template

Suggested Fix

Check if the file exists before writing:

let pre_commit_path = self._hooks_abs_dir(repo_path.as_str()).join("pre-commit");
if !pre_commit_path.exists() {
    fs::write(pre_commit_path, r#"#!/bin/sh..."#)?;
}

Or use OpenOptions with create_new(true) to fail if the file exists.