Fix Walkdir depth

From the documentation of Walkdir::min_depth():

    Set the minimum depth of entries yielded by the iterator.

    The smallest depth is 0 and always corresponds to the path given to
    the new function on this type. Its direct descendents have depth 1,
    and their descendents have depth 2, and so on.

This means that when we started with "/tmp/store", we end up yielding
that exact path in the first iteration. This is exactly what we do _not_
want.

Setting the minimal depth to 1 fixes this bug.

lib/core/libimagstore/src/file_abstraction/fs.rs

@@ -159,6 +159,7 @@ impl FileAbstraction for FSFileAbstraction {
         use walkdir::WalkDir;
 
         let i : Result<Vec<PathBuf>, SE> = WalkDir::new(basepath)
+            .min_depth(1)
             .into_iter()
             .map(|r| {
                 r.map(|e| PathBuf::from(e.path()))