内容来自：http://marklodato.github.com/visual-git-guide/index-en.html
什么是reference： reference，引用指的都是.git下的文件。
HEAD就是reference中的一个, HEAD的指向的都是current branch。
注意branch上面箭头表示可以移动到任意一个commit的id上。commit的箭头表示指向parent commit对象。
理解了reference之后，对于reset也很好理解。The reset command moves the current branch to another position。比如下图的操作表示将当前分支沿着指针指向，向ancestor移动三次。
git merge的操作中，针对Fast Forward情况有move reference的操作。
git merge 源分支，该命令的含义是把源分支的内容合并的目标分支上或者当前分支上（也就是current branch上） 比如git merge master表示要将master分支合并到current branch上来。注意，合并的结果是HEAD指向了current branch。
merge有false,true二种merge：
1） “false merge”
a.如果源分支是current branch的ancestor,那么do nothing。（current branch已经是最新的了，所以什么都不做）
b.如果current branch是源分支的ancestor，那么只做reference的move，这称为fast forward.下面是fast forward的情况。（current branch是源分支的祖先,也就意味着current branch旧，源分支新。 将新合并到旧的结果是新建了一个commit。同时将当前分支与源分支都指向这个commit）
2)“true merge”，也就是Non Fast Forward的情况出现时的merge。这种merge一般不鼓励的。因为一个commit会指向二个parent，这样的做法是导致non line-history出现。
可选多种merge策略。默认是recursive策略。
a.原理：three-way merge，而不再是简单的do nothing或者fast merge. 注意三路对应的是哪三路：takes the current commit (ed489 below), the other commit (33104), and their common ancestor (b325c), and performs a three-way merge.
b.特点：会产生一个新的commit对象。
Otherwise, a "real" merge must occur. You can choose other strategies, but the default is to perform a "recursive" merge, which basically takes the current commit (ed489 below), the other commit (33104), and their common ancestor (b325c), and performs a three-way merge. The result is saved to the working directory and the stage, and then a commit occurs, with an extra parent (33104) for the new commit.
git merge other表示将other分支做为源分支合并到current branch－master上。
当three-way merge制造了non-line history时，git rebase通过人为的调整顺序将当前分支的提交内容放到了最后
rebase的定义：
A rebase is an alternative to a merge for combining multiple branches. Whereas a merge creates a single commit with two parents, leaving a non-linear history, a rebase replays the commits from the current branch onto another, leaving a linear history. In essence, this is an automated way of performing several cherry-picks in a row.
git rebase master类似于git merge master表达的语义，表示将master分并到current branch上。(current branch已经由上图的master变成了下图的topic)
下面是需要merge或rebase的场景：
Afterward, maint is no longer an ancestor of master. To join the two histories, a merge (or rebase) will be necessary. A rebase is an alternative to a merge for combining multiple branches.
