diff: use binary diff when copy source is binary When a binary source has been copied or renamed into a non-binary file, we don't check whether the copy source was binary. There is a code comment explaining that a git mode will be forced anyway in order to capture the copy record (i.e. losedatafn() will be called). This is true, but forcing git mode is not the only effect binary files have: when git mode was already requested, we use the binary-ness to tell us whether to use a regular unified diff or a git binary diff. The user sees this as a "Binary file $file has changed" instead of the binary