Fix 'git log' early pager startup error case

We start the pager too early for several git commands, which results in
the errors sometimes going to the pager rather than show up as errors.

This is often hidden by the fact that we pass in '-X' to less by default,
which causes 'less' to exit for small output, but if you do

  export LESS=-S

you can then clearly see the problem by doing

  git log --prretty

which shows the error message ("fatal: unrecognized argument: --prretty")
being sent to the pager.

This happens for pretty much all git commands that use USE_PAGER, and then
check arguments separately. But "git diff" does it too early too (even
though it does an explicit setup_pager() call)

This only fixes it for the trivial "git log" family case.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>