diff: let "git diff -O" read orderfile from any file and fail properly

The -O flag really shouldn't silently fail to do anything when given
a path that it can't read from.

However, it should be able to read from un-mmappable files, such as:

 * pipes/fifos

 * /dev/null:  It's a character device (at least on Linux)

 * ANY empty file:

   Quoting Linux mmap(2), "SUSv3 specifies that mmap() should fail if
   length is 0.  However, in kernels before 2.6.12, mmap() succeeded in
   this case: no mapping was created and the call returned addr.  Since
   kernel 2.6.12, mmap() fails with the error EINVAL for this case."

We especially want "-O/dev/null" to work, since we will be documenting
it as the way to cancel "diff.orderfile" when we add that.

(Note: "-O/dev/null" did have the right effect, since the existing error
handling essentially worked out to "silently ignore the orderfile".  But
this was probably more coincidence than anything else.)

So, lets toss all of that logic to get the file mmapped and just use
strbuf_read_file() instead, which gives us decent error handling
practically for free.

Signed-off-by: Samuel Bronson <naesten@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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