相关概念：

1. Tree(

   Git tracks the pathnames of files through another kind of object

   called a tree.

   and might be pointed by different commits)

2. Index

3. Blob: Binary file for storge

4. Commit: according to each ci, the head point to the latest ci.  A commit might point to its parent commit。Different commits might have same trees. Ci storaged in a graph struct.

5. Tag: a tag poin to one ci

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