Merge branch 'pw/diff-no-index-from-named-pipes'

"git diff --no-index" learned to read from named pipes as if they
were regular files, to allow "git diff <(process) <(substitution)"
some shells support.

* pw/diff-no-index-from-named-pipes:
  diff --no-index: support reading from named pipes
  t4054: test diff --no-index with stdin
  diff --no-index: die on error reading stdin
  diff --no-index: refuse to compare stdin to a directory