Merge branch 'jk/output-prefix-cleanup'

Code clean-up.

* jk/output-prefix-cleanup:
  diff: store graph prefix buf in git_graph struct
  diff: return line_prefix directly when possible
  diff: return const char from output_prefix callback
  diff: drop line_prefix_length field
  line-log: use diff_line_prefix() instead of custom helper

diff --cc diff-lib.c
Simple merge

diff --cc diff.c
Simple merge

diff --cc diff.h
Simple merge

diff --cc graph.c
Simple merge

diff --cc line-log.c
Simple merge

diff --cc log-tree.c
Simple merge

diff --cc range-diff.c
Simple merge