combine-diff.c: teach combined diffs about line prefix

combine-diff.c: teach combined diffs about line prefix

When running "git log --graph --cc -p" the diff output for merges is not
indented by the graph structure, unlike the diffs of non-merge commits
(added in commit 7be5761 - diff.c: Output the text graph padding before
each diff line).

Fix this by teaching the combined diff code to output diff_line_prefix()
before each line.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>