Running "blame parent commit" currently brings up an empty blame view
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: NMartin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: NPaul Mackerras <paulus@samba.org>