In some situations it is useful to be able to switch viewers via the
environment, e.g. in Emacs shell buffers. So check the GIT_MAN_VIEWER
environment variable and try it before falling back to "man".
Signed-off-by: Romain Francoise <romain@orebokech.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>