try retrieval from more than one export location when the first fails

try retrieval from more than one export location when the first fails

Combined with commit 0ffc59d34107a3ee671b90fc7c59ca09c00abbf5, this
fixes the case where there are duplicate files on the special remote,
and the first gets modified/deleted, while the second is still present.

directory, adb: Fixed a bug when importtree=yes, and multiple files in the
special remote have the same content, that caused it to refuse to get a
file from the special remote, incorrectly complaining that it had changed,
due to only accepting the inode+mtime of one file (that was since modified
or deleted) and not accepting the inode+mtime of other duplicate files.

Sponsored-by: Max Thoursie on Patreon