git-cmd: set the TERM appropriately to avoid warnings from 'less'

This fixes issue #98 by applying the change used in commit 4d0911d to the
batch script used to start a PortableGit shell. Ensuring the TERM variable
is set avoids a "WARNING: terminal is not fully functional" warning from
less.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>