git-svn: "git worktree" awareness

git-svn: "git worktree" awareness

git-svn internals were previously not aware of repository
layout differences for users of the "git worktree" command.
Introduce this awareness by using "git rev-parse --git-path"
instead of relying on outdated uses of GIT_DIR and friends.

Thanks-to: Duy Nguyen <pclouds@gmail.com>
Reported-by: Mathieu Arnold <mat@freebsd.org>
Signed-off-by: Eric Wong <e@80x24.org>