gitk: Fix "External diff" with separate work tree

gitk: Fix "External diff" with separate work tree

Running "External diff" to compare the index and work tree currently
brings up an empty blame view when the work tree is not the parent of
the git directory.  This is because the file that is taken from the
work tree is assumed to be in
$GIT_DIR/../<repo-relative-file-name>.  Fix it by feeding the diff tool
a path under $GIT_WORK_TREE instead of "$GIT_DIR/..".

Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>