[PATCH 7/7] diff: avoid useless filespec population

builtin_diff calls fill_mmfile fairly early, which in turn calls diff_populate_filespec, which actually retrieves the file's blob contents into a buffer. Long ago, this was sensible as we would need to look at the blobs eventually.

These days, however, we may not ever want those blobs if we end up using a textconv cache, and for large binary files (exactly the sort for which you might have a textconv cache), just retrieving the objects can be costly.

This patch just pushes the fill_mmfile call a bit later, so we can avoid populating the filespec in some cases. There is one thing to note that looks like a bug but isn't. We push the fill_mmfile down into the first branch of a conditional. It seems like we would need it on the other branch, too, but we don't; fill_textconv does it for us (in fact, before this, we were just writing over the results of the fill_mmfile on that branch).

Here's a timing sample on a commit with 45 changed jpgs and avis. The result is fully textconv cached, but we still wasted a lot of time just pulling the blobs from storage. The total size of the blobs (source and dest) is about 180M.

  [before]
  $ time git show >/dev/null
  real    0m0.352s
  user    0m0.148s
  sys     0m0.200s

  [after]
  $ time git show >/dev/null
  real    0m0.009s
  user    0m0.004s
  sys     0m0.004s

And that's on a warm cache. On a cold cache, the "after" case is not much worse, but the "before" case has to do an extra 180M of I/O.

Signed-off-by: Jeff King <peff@peff.net>
---
I remember when I first started using git having it go so fast that I
had to double-check that it had actually worked correctly. I had another
of those moments when I ran this. But it correctly processed everything
I threw at it. More stress-testing appreciated. :)

 diff.c |    9 ++++-----
 1 files changed, 4 insertions(+), 5 deletions(-)