Improve "git log --graph" output of merges

This series changes a couple of places that do not currently indent their output when being shown with a graph.

The first patch was already posted [1] and addresses the output of "git log --graph -c -p". Patch 2 is an independent fix I noticed while working on the later patches.

Patches 3-5 introduce a helper function and change existing sites using diff_options->output_prefix to use it, resulting in a net reduction by about 60 lines. There is no functional change here.

The final patch uses the helper introduced in patch 4 to make combined diffs should the output prefix before each line. This affects the output of "git log --graph --cc [-p|--raw]".

[1] http://article.gmane.org/gmane.comp.version-control.git/215630

John Keeping (6):
graph: output padding for merge subsequent parents
diff: write prefix to the correct file diff.c: make constant string arguments const
diff: add diff_line_prefix function
diff.c: use diff_line_prefix() where applicable combine-diff.c: teach combined diffs about line prefix

 combine-diff.c |  47 +++++++++++++--------
 diff.c         | 131 +++++++++++++++------------------------------------------
 diff.h         |   3 ++
 graph.c        |  10 +++++
 4 files changed, 77 insertions(+), 114 deletions(-)

1.8.1.2