Where a change set comes from: --files <path|-> or --base <git-ref>.

From git, the change set is the union of

• git diff --name-only --no-relative --no-renames <ref>...HEAD (committed on the branch)
• git diff --name-only --no-relative --no-renames HEAD (staged and unstaged)
• git ls-files --others --exclude-standard --full-name -- :/ (untracked)

All three cover the whole repository, not only the project directory: a diff.relative config is overridden and untracked files are listed from the top level, so a project in a subdirectory still sees changes beside it.

--no-renames makes a moved file show as delete + add, so its old path forces a full run as an unmapped file. Paths are made relative to the project root (git reports them relative to the repository top level); a changed file outside the project root stays absolute, which the classifier turns into a full run.

The ref is checked before any diff: an empty or blank ref (git would read ...HEAD as an empty range), an option-like ref (git would read it as an option, e.g. --output=<file>) and a ref that does not resolve to a commit are errors, never an empty change set. Every diff ends its revisions with --, so a ref is never read as a path. A ref with no merge base with HEAD (an unrelated history, or a shallow clone missing it) is an error.