range-diff: add headers to the outer hunk header

range-diff: add headers to the outer hunk header

Add the section headers/hunk headers we introduced in the previous
commits to the outer diff's hunk headers.  This makes it easier to
understand which change we are actually looking at.  For example an
outer hunk header might now look like:

    @@  Documentation/config/interactive.txt

while previously it would have only been

    @@

which doesn't give a lot of context for the change that follows.

For completeness also add section headers for the commit metadata and
the commit message, although they are arguably less important.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>