build several books in parallel

This patch makes it possible to run several builds of affected
books in parallel.

Change-Id: I155ee1fa3fc7ebcf53a0b00558b5a76eabf4ad8d
Update Sourcetree download link to use link tags

Change-Id: Ifdf8551d6c49ee50612ca0d9e46557267e39b90a
Fixes: Bug1212477
Update Git download location to the correct URL

Change-Id: I7950b5cb34c1a3759d0677cbfc3226b2a67840d0
Merge "build several books in parallel"
Merge "Update Git download location to the correct URL"
Merge "Update Sourcetree download link to use link tags"