`$rspfile` in command is not evaluated properly

[hikerpig]

For example, when I run n2 test and check rspoutput.txt.

Expected to see 'test.rsp' , this is what ninja test generates.

But actually with n2, I got an empty rspoutput.txt file.

build build.ninja.stamp: phony
  generator = 1
  depfile = build.ninja.d

rule echo_rsp
  rspfile = test.rsp
  rspfile_content = some texts
  command = echo $rspfile > rspoutput.txt

build test: echo_rsp

[Colecf]

We discussed this in #94, Evan was hesitant to add this behavior, and in my case, updating Android to not rely on it was simple enough.