Should export GIT_PAGER in `vsh_shell_start`

[mmalcomson]

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There are a bunch of configuration things that make working with vsh much nicer.

Things like set pagination off in GDB etc. One of these is GIT_PAGER='col -b'. I think that should be set in vsh_shell_start

N.b. if I intend to make this more widely known, then I should put a bit more effort into the "introductory session" thing that I was working on a little while ago -- i.e. https://github.com/hardenedapple/vsh/blob/master/demo/setup-env.vsh?plain=1