Don't scan uncompressed_objects_dir if it doesn't exist

Don't scan uncompressed_objects_dir if it doesn't exist

A newly created archive-mode repository won't have a uncompressed-objects-cache
directory, and uncompressed_objects_dir is -1 to flag that. The special meaning of
-1 meaning "cwd" for libglnx means that the current directory was scanned as
if it was an objects directory, producing unexpected results, especially if there
were any two-letter files/subdirs in the current directory.

Closes: #1537
Approved by: jlebon