Given that sparse file logic is not trivial, there should be a test
in TestPartialRead to ensure that partial reads work.

Change-Id: I913da3e331da06dca6758a8be3f5099abba233a6
Reviewed-on: https://go-review.googlesource.com/54430
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>