fix(args): scope --workdir to its directory on Windows - #1657

Description

On Windows, git cliff --workdir <subdir> doesn't scope the changelog to that directory; every commit in the repository is listed.

Two things go wrong in the include pattern built from --workdir (added in #1627):

1. fs::canonicalize returns a \?\C:\... verbatim path on Windows, while the repository root reported by libgit2 is a plain C:/... path. strip_prefix fails, so no include pattern is added at all.
2. With that fixed, the pattern is built with to_string_lossy(), which gives sub\dir\. normalize_pattern only appends ** after a trailing / (since fix(git): match directory include/exclude patterns on Windows #1593), and git diff paths always use /, so the pattern would match nothing and the changelog would be empty.

This moves the logic into a small workdir_include_pattern helper that canonicalizes both the root and the workdir, and builds the pattern from the path components joined with /. On Linux and macOS the resulting pattern is the same as before.

#1379 also canonicalizes paths, but it still builds the pattern with to_string_lossy(), so it wouldn't cover the second part.

Motivation and Context

Repository with three commits, only one touching sub/:

git cliff --workdir sub

Before (Windows): all three commits are listed. After: only the commit that touches sub/. --workdir . still lists everything, and running from inside sub/ without --workdir is unchanged.

How Has This Been Tested?

Added two unit tests for workdir_include_pattern. On Windows, workdir_include_pattern_is_repo_relative_with_forward_slashes fails without the change and passes with it. cargo test -p git-cliff passes on Windows 11 (x86_64-pc-windows-gnu), and the repository above gives the output described. The test job runs on Linux only, where the test passes both before and after.

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (no code change)
• Refactor (refactoring production code)
• Other

Checklist:

• My code follows the code style of this project.
• I have updated the documentation accordingly (if applicable).
• I have formatted the code with rustfmt.
  • cargo +nightly fmt --all
• I checked the lints with clippy.
  • cargo clippy --tests --verbose -- -D warnings
• I have added tests to cover my changes.
• All new and existing tests passed.
  • cargo test

I used Claude Code to write this change.