pre-commit — Notification just prior to commit completion.
The pre-commit hook is run just
before a commit transaction is promoted to a new revision.
Typically, this hook is used to protect against commits that
are disallowed due to content or location (e.g., your
site might require that all commits to a certain branch
include a ticket number from the bug tracker, or that the
incoming log message is nonempty).
If the pre-commit hook program
returns a nonzero exit value, the commit is aborted, the
commit transaction is removed, and anything printed to
stderr is marshalled back to the client.