Don't reallocate when capacity is already equal to length

`Vec::shrink_to_fit()` may be called on vectors that are already the
correct length. Calling out to `reallocate()` in this case is a bad idea
because there is no guarantee that `reallocate()` won't allocate a new
buffer anyway, and based on performance seen in external benchmarks, it
seems likely that it is in fact reallocating a new buffer.

Before:

    test string::tests::bench_exact_size_shrink_to_fit         ... bench:        45 ns/iter (+/- 2)

After:

    test string::tests::bench_exact_size_shrink_to_fit         ... bench:        26 ns/iter (+/- 1)

src/libcollections/vec.rs

@@ -356,7 +356,7 @@ impl<T> Vec<T> {
                 }
                 self.cap = 0;
             }
-        } else {
+        } else if self.cap != self.len {
             unsafe {
                 // Overflow check is unnecessary as the vector is already at
                 // least this large.