Merge branch 'jk/diff-graph-cleanup'

Merge branch 'jk/diff-graph-cleanup'

Refactors a lot of repetitive code sequence from the graph drawing
code and adds it to the combined diff output.

* jk/diff-graph-cleanup:
  combine-diff.c: teach combined diffs about line prefix
  diff.c: use diff_line_prefix() where applicable
  diff: add diff_line_prefix function
  diff.c: make constant string arguments const
  diff: write prefix to the correct file
  graph: output padding for merge subsequent parents