Magit comes with the minor mode
git-commit-mode, enabled via the entry-point
global-git-commit-mode, which runs the hook and user option
git-commit-setup-hook as its last step. You can add your
git-commit-mode-specific setup code to this as you would any other hook variable:
(add-hook 'git-commit-setup-hook #'my-git-commit-setup-yasnippet)
You could also use the Customize interface to achieve this via
M-x customize-option RET git-commit-setup-hook RET. Either way I recommend you look at the documentation and suggested and already enabled hook functions reported by Customize for this hook.
The setup code for
git-commit-mode also enables an arbitrary major mode which controls the overall editing behaviour for commit messages. This mode is determined by the user option
git-commit-major-mode. Thus the hook for whichever major mode you specify could offer another means of running custom setup code.