diff: add --binary option for git mode diffs
This patch adds --binary option to `hg diff` and `hg export` to allow more
control about when binary diffs are displayed in Git mode as well as some
tests to verify it behaves correctly (issue5510).
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https://www.mercurial-scm.org/wiki/ContributingChanges
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273ce12ad8f1` will report some style violations on a very old commit).