[Buildbot] #2945: Add an example of using codebases

[Buildbot trac]

#2945: Add an example of using codebases
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Reporter:  sa2ajj            |       Owner:
    Type:  enhancement       |      Status:  new
Priority:  patches-accepted  |   Milestone:  0.9.+
 Version:  master            |  Resolution:
Keywords:  docs              |
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Comment (by Ben):

 I could help there. Just need an example where it makes sense to use
 codebases: i.e. It is necessary to checkout multiple repository in order
 to run a command that need the outcome of each of them.

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Ticket URL: <http://trac.buildbot.net/ticket/2945#comment:1>
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