mail.sh: allow working without an shlib.sh
Allow the mail.sh script to be run without needing to
have a fully configured and installed shlib.sh file.
Since mail.sh is originally derived from the
post-receive-email hook script it was initially
intended to work without any shlib.sh in the first
place.
With only the very tiniest of adaptations, it can do
so again.
Having the ability to run mail.sh without needing an
installed and configured shlib.sh greatly facilitates
the testing of various ref change scenarios.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>