range-diff: add filename to inner diff

range-diff: add filename to inner diff

In a range-diff it's not always clear which file a certain funcname of
the inner diff belongs to, because the diff header (or section header
as added in a previous commit) is not always visible in the
range-diff.

Add the filename to the inner diffs header, so it's always visible to
users.

This also allows us to add the filename + the funcname to the outer
diffs hunk headers using a custom userdiff pattern, which will be done
in the next commit.

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