Following 'customary' practice for Linux in my own profile.

See here for reference:
    https://coderwall.com/p/u003pa
which also points out a distinction between the behavior of the Mac OS
X Terminal.app from most/all Linux terminal emulators.

My main motivation for doing this is that I want my customized prompt
(with current time and git/hg branch info) to be the default when I
start a terminal session.