Backport #331 - Asset lookup 4x Faster in large directories

With this patch using the benchmark script https://github.com/schneems/sprockets-3.x-performance-regressions:

```
1500 lookups of the same asset took 2.900005
1500 lookups of 1500 different assets took 2.746331
```

Without this patch using the benchmark script:

```
1500 lookups of the same asset took 12.361615
1500 lookups of 1500 different assets took 12.627099
```

Which is a 4x speed bump.

Author: schneems

lib/sprockets/resolve.rb

@@ -178,6 +178,7 @@ def dirname_matches(dirname, basename)
         candidates = []
         entries = self.entries(dirname)
         entries.each do |entry|
+          next unless File.basename(entry).start_with?(basename)
           name, type, _, _ = parse_path_extnames(entry)
           if basename == name
             candidates << [File.join(dirname, entry), type]