git-diff-x invoked from a subdirectory with a ..-containing path
(e.g. gdx <cmd> ../foo) failed because the path was passed verbatim
to git show <ref>:<path>, which doesn't normalize .. segments.
Fix: normalize the joined <prefix><path> via posixpath.normpath
before handing it to git show. Worktree side keeps the original
path (shell handles .. fine).
Full Changelog: v0.1.5...v0.1.6