track "assumed" filesystem usage (#89)

As described in #89, we need to refactor a bit before we can get the
actual filesystem block size. Assuming 4096 for now. Small steps.

base/strutil.rs

@@ -44,7 +44,7 @@ static MULTIPLIERS: [(char, u64); 4] = [
     ('K', 10),
 ];
 
-/// Encodes a size into human-readable form.
+/// Encodes a non-negative size into human-readable form.
 pub fn encode_size(mut raw: i64) -> String {
     let mut encoded = String::new();
     for &(c, n) in &MULTIPLIERS {