diff: do not reuse worktree files that need "clean" conversion

When accessing a blob for a diff, we may try to reuse file contents in the working tree, under the theory that it is faster to mmap those file contents than it would be to extract the content from the object database. When we have to filter those contents, though, that assumption does not hold. Even for our internal conversions like CRLF, we have to allocate and fill a new buffer anyway. But much worse, for external clean filters we have to exec an arbitrary script, and we have no idea how expensive it may be to run. So let's skip this optimization when conversion into git's "clean" form is required. This applies whenever the "want_file" flag is false. When it's true, the caller actually wants the smudged worktree contents, which the reused file by definition already has (in fact, this is a key optimization going the other direction, since reusing the worktree file there lets us skip smudge filters). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>