Merge branch 'dd/diff-customize-index-line-abbrev'

Merge branch 'dd/diff-customize-index-line-abbrev'

The output from the "diff" family of the commands had abbreviated
object names of blobs involved in the patch, but its length was not
affected by the --abbrev option.  Now it is.

* dd/diff-customize-index-line-abbrev:
  diff: index-line: respect --abbrev in object's name
  t4013: improve diff-post-processor logic