Git limits its tree diff by the pathspec before it looks for renames, so
a directory only ever gets one end of a rename whose other end is outside
it. Nothing is left to pair up, and the file turns into an addition or a
deletion that the commit doesn't contain.
Pass the other end along with the directory. This is bounded by the
number of renames that cross the directory's boundary, so it costs
nothing at all for the vast majority of commits.
The files and commit files panels each had their own copy of this, one of
which used to be missing the previous path of a rename. Growing them
apart again is the last thing we want, since the next commit needs to
teach both of them about renames that cross a directory boundary.
The files panel version only returned paths for the filtered case, and
left it to WorktreeFileDiffCmdObj to derive the rest from the node; now
that all callers pass the paths in, that command doesn't need to know
about renames at all.
