lib/sort.c optimization

Hello, I fixed and tested a small bug in lib/sort.c file, heap sort
function.

The fix avoids unnecessary swap of contents when i is 0 (saves few loads
and stores), which happens every time sort function is called.  I felt the
fix is worth bringing it to your attention given the importance and
frequent use of the sort function.

Acked-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>