Improve DiffFormatter text file access

Improve DiffFormatter text file access

When we are asked to create a difference between two files the caller
really wants to see that output.  Instead of punting because a file
is too big to process, consider it to be binary.  This reduces the
accuracy of our output display, but makes it a lot more likely that
the formatter can still generate something semi-useful.

We set our default binary threshold to 50 MiB, which is the same
threshold that PackWriter uses before punting and deciding a file
is too big to delta compress.  Anything under this size we try to
load and process, anything over that size (or that won't allocate
in the heap) gets tagged as binary.

Change-Id: I69553c9ef96db7f2058c6210657f1181ce882335
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>