was_dirty() uses was_tracked(), which has been updated to use the original
index rather than the current one. However, was_dirty() also had a
separate call to cache_file_exists(), causing it to still implicitly use
the current index. Update that to instead use index_file_exists().
Also, was_dirty() had a hack where it would mark any file as non-dirty if
we simply didn't know its modification time. This was due to using the
current index rather than the original index, because D/F conflicts and
such would cause unpack_trees() to not copy the modification times from
the original index to the current one. Now that we are using the original
index, we can dispense with this hack.
Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>