git-svn: Simplify calculation of GIT_DIR

git-svn: Simplify calculation of GIT_DIR

Since git-rev-parse already checks for the $GIT_DIR environment
variable and that it returns an actual git repository, there is no
need to repeat the checks again here.

This also fixes a problem where git-svn did not work in cases where
.git was a file with a gitdir: link.

[ew: squashed test case,
 delay setting GIT_DIR until after `git rev-parse --cdup` to fix t9101,
 (thanks to Junio)]

Signed-off-by: Barry Wardell <barry.wardell@gmail.com>
Signed-off-by: Eric Wong <normalperson@yhbt.net>