text: Title: Refactoring procs/pipelines.py
  Body: This is metaissue for list of tasks on refactoring.
  
  Refactoring procs/pipelines.py:
  * Rename `CommandPipeline.proc` to `last_proc` to represent the actual thing that code is really doing.
  * Show the whole pipeline in `CommandPipeline.__repr__` instead of last proc.
  * Move all logic about decisions to specs build. It's also needed to show right `$XONSH_TRACE_SUBPROC=2`.
  
  
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