diff: add a fast path to avoid loading binary contents
When diffing binary contents, with certain configs, we can show
"Binary file <name> has changed" without actual content.
That allows a fast path where we could avoid providing actual binary
contents. Note: in that case we still need to test if two contents are the
same, that's done by using "filectx.cmp", which could have its own fast
path.
Mercurial
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