Under core Git the git-commit tool will invoke the commit-msg hook if it exists and is executable to the user running git-commit. As a hook it has some limited value as it cannot alter the commit, but it can modify the message the user is attempting to commit. It is also able to examine the message to ensure it conforms to some local standards/conventions.
Since the hook takes the name of a temporary file holding the message as its only parameter we need to move the code that creates the temp file up earlier in our commit code path, and then pass through that file name to the latest stage (where we call git-commit-tree). We let the hook alter the file as it sees fit and we don't bother to look at its content again until the commit succeeded and we need the subject for the reflog update.
Signed-off-by: Shawn O. Pearce <email@example.com>