git-merge有两种merge方式，ff方式和true merge方式，关于ff的方式，另外一篇文章有讲过，这里不再赘述，这里整理一下true merge，真实merge。
Except in a fast-forward merge (see above), the branches to be merged must be tied together by a merge commit that has both of them as its parents.
A merged version reconciling the changes from all branches to be merged is committed, and your HEAD, index, and working tree are updated to it. It is possible to have modifications in the working tree as long as they do not overlap; the update will preserve them.
When it is not obvious how to reconcile the changes, the following happens:
The HEAD pointer stays the same.
The MERGE_HEAD ref is set to point to the other branch head.
Paths that merged cleanly are updated both in the index file and in your working tree.
For conflicting paths, the index file records up to three versions: stage 1 stores the version from the common ancestor, stage 2 from HEAD, and stage 3 from MERGE_HEAD (you can inspect the stages with git ls-files -u). The working tree files contain the result of the “merge” program; i.e. 3-way merge results with familiar conflict markers <<< === >>>.
No other changes are made. In particular, the local modifications you had before you started merge will stay the same and the index entries for them stay as they were, i.e. matching HEAD.
If you tried a merge which resulted in complex conflicts and want to start over, you can recover with git merge –abort.
关于HEAD、index、worktree、local repository、remote repository的关系，请参考这里，这个挺重要，随后要整理一下。
The merge mechanism (git merge and git pull commands) allows the backend merge strategies to be chosen with -s option. Some strategies can also take their own options, which can be passed by giving -X