file: Improve performance of has_same_content by spawning cmp less frequently

* If the two sizes are different, files are different
* Else read the first SMALL_FILE_THRESHOLD bytes (64K)
  * If the first bytes are different, files are different
  * Else first bytes are identical, and if file size is
    <= SMALL_FILE_THRESHOLD then files are identical

Only if all these checks fail, spawn external command `cmp -s` to do the
comparison.