Make "git grep" and "git show" pay attention to --textconv when
dealing with blob objects.

* mg/more-textconv:
  grep: honor --textconv for the case rev:path
  grep: allow to use textconv filters
  t7008: demonstrate behavior of grep with textconv
  cat-file: do not die on --textconv without textconv filters
  show: honor --textconv for blobs
  diff_opt: track whether flags have been set explicitly
  t4030: demonstrate behavior of show with textconv