add reverse iteration for Cmd (JuliaLang#43498)

Author: stevengj

base/process.jl

@@ -654,6 +654,7 @@ show(io::IO, p::Process) = print(io, "Process(", p.cmd, ", ", process_status(p),
 for f in (:length, :firstindex, :lastindex, :keys, :first, :last, :iterate)
     @eval $f(cmd::Cmd) = $f(cmd.exec)
 end
+Iterators.reverse(cmd::Cmd) = Iterators.reverse(cmd.exec)
 eltype(::Type{Cmd}) = eltype(fieldtype(Cmd, :exec))
 for f in (:iterate, :getindex)
     @eval $f(cmd::Cmd, i) = $f(cmd.exec, i)

test/spawn.jl

@@ -599,6 +599,7 @@ end
 # accessing the command elements as an array or iterator:
 let c = `ls -l "foo bar"`
     @test collect(c) == ["ls", "-l", "foo bar"]
+    @test collect(Iterators.reverse(c)) == reverse!(["ls", "-l", "foo bar"])
     @test first(c) == "ls" == c[1]
     @test last(c) == "foo bar" == c[3] == c[end]
     @test c[1:2] == ["ls", "-l"]