Drop `pathdiff` dependency for GitLab output

[ntBre]

As noted in #20117, we only use pathdiff for one relativize_path_to call in our GitLab output format. This was added in #3475 to make the GitLab output relative to the CI_PROJECT_DIR environment variable. According to the docs, this is a pre-defined variable with this description:

The full path the repository is cloned to, and where the job runs from. If the GitLab Runner builds_dir parameter is set, this variable is set relative to the value of builds_dir. For more information, see the Advanced GitLab Runner configuration.

In my simple test repo, echoing $CI_PROJECT_DIR and pwd shows that the job starts out in this directory, as stated above:

$ echo $CI_PROJECT_DIR
/builds/nothanks3/test-gitlab-output-format
$ pwd
/builds/nothanks3/test-gitlab-output-format
$ ruff --version
ruff 0.12.8
$ ruff check --output-format=gitlab > code-quality-report.json

For basic usage like this, it seems fine then to drop relativize_path_to and pathdiff in favor of the fallback handling that makes the path relative to the CWD. In fact, this was the behavior before #1837, which made GitLab paths absolute. I'm just slightly concerned that this could break users with more adventurous directory changes in their CI jobs.

We could also use our own relativize_path function in render.rs, but the behavior seems to be a bit different from pathdiff::diff_paths if the base directory isn't a prefix of the path. diff_paths will add .. components in this case.

This is the only function we use from pathdiff, so yet another option is vendoring just that.