Running "show origin of this line" currently fails when the the work
tree is not the parent of the git directory. Fix it by feeding
git-blame paths relative to $GIT_WORK_TREE instead of "$GIT_DIR/..".
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com> Signed-off-by: Paul Mackerras <paulus@samba.org>