hooks: add a hook to reject merge commits

Fixes #4.
6 jobs for reject-merges-hook
Status Job ID Name Coverage
  External
passed kwrobot-branch-check

failed rust-git-hooks-megas-linux-debug

failed rust-git-hooks-megas-linux-debug

failed rust-git-hooks-megas-linux-debug

failed rust-git-hooks-megas-linux-debug

failed rust-git-hooks-megas-linux-debug

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