FileSystem: optimize lookup for last DirEnt

We do directory lookups in context of iterating over tarballs, which are
usually sorted.  We store our DirEnts sorted too.  It's therefore very
likely that if we're inserting an item, it'll be at the end of the
array, and if we're looking for a directory to perform operations inside
of, it's probably going to be the last DirEnt.  That means that instead
of starting our binary search on the middle element, we want to start
with the last one.

This is something like a ~10% improvement on the time it takes to
compose the merged filesystem tree — in-memory — and brings the 'create
a dumpfile for a ~250k files multi-layer container image' usecase from
consistently slightly more than 0.5s to consistently slightly less than
0.5s.

src/image.rs

@@ -1,4 +1,8 @@
 use std::{
+    cmp::{
+        Ord,
+        Ordering,
+    },
     ffi::{
         OsStr,
         OsString,
@@ -60,10 +64,20 @@ pub struct DirEnt {
 
 impl Directory {
     pub fn find_entry(&self, name: &OsStr) -> Result<usize, usize> {
-        // performance TODO: on the first pass through we'll almost always want the last entry
-        // (since the layer is sorted and we're always inserting into the directory that we just
-        // created) maybe add a special case for that?
-        self.entries.binary_search_by_key(&name, |e| &e.name)
+        // OCI layer tarballs are typically sorted, with the entries for a particular directory
+        // written out immediately after that directory was created.  That means that it's very
+        // likely that the thing we're looking for is either the last entry or the insertion point
+        // immediately following it.  Fast-path those cases by essentially unrolling the first
+        // iteration of the binary search.
+        if let Some(last_entry) = self.entries.last() {
+            match name.cmp(&last_entry.name) {
+                Ordering::Equal => return Ok(self.entries.len() - 1), // the last item, indeed
+                Ordering::Greater => return Err(self.entries.len()),  // need to append
+                Ordering::Less => self.entries.binary_search_by_key(&name, |e| &e.name)
+            }
+        } else {
+            Err(0)
+        }
     }
 
     pub fn recurse<'a>(&'a mut self, name: &OsStr) -> Result<&'a mut Directory> {