https://www.youtube.com/watch?v=WH6XJvUbxTI&list=PLuHgQVnccGMA8iwZwrGyNXCGy2LAAsTXk&index=28&t=62s
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git init
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vim f1.txt
a
git add f1.txt
git commit -m "1"
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vim f1.txt
git commit -am "2"
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git branch exp
git branch
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rm .git/refs/heads/exp
git branch
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git log
vim .git/refs/heads/exp
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git branch
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git checkout exp
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vim f2.txt
a
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git add f2.txt
git commit -m "3"
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git checkout master
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git log --branches --decorate --graph