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 <a51dda7c7ff50b61eaea0444371f4a6a9301e501@keeping.me.uk>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git