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