v.io/jiri/runutil: improvements inspired by caprita:-)

- add a method to both run a command and terminate the
  sequence. I've called it Last, but I'm not entirely
  convinced by this.
- the context.Seq() method creates a new sequence
  everytime it is called since this is less error prone
  than always using a shared instance.
- in non-verbose mode, only commands that generate an error
  will have their output kept, all other output is suppressed.

MultiPart: 1/2

Change-Id: Id379cefc78e9ec7e5ab43ba4fd6880c8ce1eca46
4 files changed