cmd-index: correct reporting of deleted vs. added vs. modified status.

cmd-index: correct reporting of deleted vs. added vs. modified status.

A file with an all-zero sha1 is considered Added instead of Modified, since
it has obviously *never* had a valid sha1.  (A modified file has an old
sha1, but IX_HASHVALID isn't set.)

We also now don't remove old files from the index - for now - so that we can
report old files with a D status.  This might perhaps be useful eventually.

Furthermore, we had a but where reindexing a particular filename would
"sometimes" cause siblings of that file to be marked as deleted.  The
sibling entries should never be updated, because we didn't check them and
thus have no idea of their new status.  This bug was mostly caused by the
silly way we current pass dirnames and filenames around...