use importChanges optimisation

use importChanges optimisation

Large speed up to importing trees from special remotes that contain a lot
of files, by only processing changed files.

Benchmarks:

Importing from a special remote that has 10000 files, that have all been
imported before, and 1 new file sped up from 26.06 to 2.59 seconds.

An import with no change and 10000 unchanged files sped up from 24.3 to
1.99 seconds.

Going up to 20000 files, an import with no changes sped up from
125.95 to 3.84 seconds.

Sponsored-by: k0ld on Patreon