git-svn: t/t91??-*: optimize the tests a bit

git-svn: t/t91??-*: optimize the tests a bit

This removes some unnecessary 'svn up' calls throughout

t9103-git-svn-graft-branches.sh:
  * removed an 'svn log' call that was leftover from debugging
  * removed multiple git-svn calls with a multi-init / multi-fetch
    combination (which weren't tested before, either)
  * replaced `rev-list ... | head -n1` with `rev-parse ...`
    (not sure what I was thinking when I wrote that)

All this saves about 9 seconds from a test run
(53s -> 44s for 'make t91*') on my 1.3GHz Athlon

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>