Merge branch 'ap/diff-ignore-blank-lines'

Merge branch 'ap/diff-ignore-blank-lines'

"git diff" learned a mode that ignores hunks whose change consists
only of additions and removals of blank lines, which is the same as
"diff -B" (ignore blank lines) of GNU diff.

* ap/diff-ignore-blank-lines:
  diff: add --ignore-blank-lines option