[FS#361] git: add pre-receive hook to reject commits without SoB line

[LEDE Bugs]

The following task has a new comment added:

FS#361 - git: add pre-receive hook to reject commits without SoB line
User who did this - Jo-Philipp Wich (jow-)

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Implemented now for web.git, source.git, keyring.git and buildbot.git
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=361#comment1142