grep: load file data after checking binary-ness

Usually we load each file to grep into memory, check whether
it's binary, and then either grep it (the default) or not
(if "-I" was given).

In the "-I" case, we can skip loading the file entirely if
it is marked as binary via gitattributes. On my giant
3-gigabyte media repository, doing "git grep -I foo" went
from:

  real    0m0.712s
  user    0m0.044s
  sys     0m4.780s

to:

  real    0m0.026s
  user    0m0.016s
  sys     0m0.020s

Obviously this is an extreme example. The repo is almost
entirely binary files, and you can see that we spent all of
our time asking the kernel to read() the data. However, with
a cold disk cache, even avoiding a few binary files can have
an impact.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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