samples for libgit2

更具体的说明: libgit2使用教程
基础术语详解

sample1: git init
sample2: git add
sample3: git commit
sample4: git status
sample5: git diff
sample6: git branch
sample7: git merge
sample8: git remote
sample9: git fetch & pull
sample10: git clone
sample11: git push
sample12: git tag
sample13: git rebase