import: Sped up import from special remote when the imported tree is unchanged

import: Sped up import from special remote when the imported tree is unchanged

I saw a nearly 2 minute speed up from this, in a repo with 56000 files some
of which are preferred content of the special remote and others not. In
such a case, addBackExportExcluded has to do a lot of work, which is
unncessary when the tree is unchanged.

When using sync --content, preferred content checking of that many files
takes about 1 minute. So this speeds up sync --content by 3x.
When using git-annex import, the speed up is much larger.

Sponsored-by: Nicholas Golder-Manning on Patreon