mdiff: distinguish diff headers from hunks in unidiff()
Let unidiff return the list of headers it produces (lines '--- <original>' and
'+++ <new>') apart from diff hunks. In patch.diff(), we combine headers
generated there (not specific to unified format) with those from unidiff().
By returning a list of header lines, we do not append new lines in datetag
inner function of unidiff() so that all header lines are '\n'.join-ed in a
similar way.
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