pager: factor out a helper to prepare a child process to run the pager

pager: factor out a helper to prepare a child process to run the pager

When running a pager, we need to run the program git_pager() gave
us, but we need to make sure we spawn it via the shell (i.e. it is
valid to say PAGER='less -S', for example) and give default values
to $LESS and $LV environment variables.  Factor out these details
to a separate helper function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>