Git commit generation number

Git Commit Generation Numbers - Stack Overflow
http://stackoverflow.com/questions/6702821/git-commit-generation-numbers

[RFC/PATCHv2 3/6] commit: add commit_generation function -- Git
http://www.spinics.net/lists/git/msg161165.html

[PATCH] Add a 'generation' number to commits -- Git
http://www.spinics.net/lists/git/msg161311.html

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[wiki] Directed acyclic graph (DAG)
http://en.wikipedia.org/wiki/Directed_acyclic_graph