关于Git Merge时的--ff 和 --no-ff 参数，ff代表的意思是fast forward，即直接快速前进。
man git-merge的解释:
--ff
When the merge resolves as a fast-forward, only update the branch pointer, without creating a merge commit. This is the default behavior.
--no-ff
Create a merge commit even when the merge resolves as a fast-forward. This is the default behaviour when merging an annotated (and possibly signed)
tag.
fast forward 和 no fast forward 的合并图:
(图片来至 A successful Git branching model)
fast forward 快速前进的合并方式适用于分支B从分支A从checkout出来后，分支A没有commit，这样分支B被合并到分支A时，可以快速前进，这个也是git默认的合并方式。
如果分支B被checkout出来后，分支A也有修改，那么就没法快速前进合并，会额外建立一个merge commit，对分支A和分支B做一个合并操作。
使用 no fast forward 的好处就是时分支的结构性没有做改动，保持分支的结构，把分支B的多次开发最后做一个merge commit 合并到分支A上。
网上有一些探讨的帖子和文章，建议把--no-ff作为默认的方式，比如上面提到的 A successful Git branching model。
另外，StackOverflow上也有几篇很详细的讲解:
- What is the difference between ‘git merge’ and ‘git merge –no-ff’?
- Git fast forward VS no fast forward merge
- Why does git fast-forward merges by default?
- When to use the ‘–no-ff’ merge option in Git
- Fast-Forward Git Merge
首先checkout一个分支，叫ff, 并切换过去:
TankyWoo@Mac::test-git/ (master) » git co -b ff master
Switched to a new branch 'ff'
新建一个文件c，做一个commit，再随便修改下，并又做一个commit。
接着切换回master分支，做 fast forward 合并:
TankyWoo@Mac::test-git/ (ff) » git co master
Switched to branch 'master'
TankyWoo@Mac::test-git/ (master) » git merge ff
Updating 2f1e828..8bffa7b
Fast-forward
c | 1 +
1 file changed, 1 insertion(+)
create mode 100644 c
查看log:
* 8bffa7b - (HEAD, ff) update c (10 seconds ago) <Tanky Woo>
* 525481f - add c (18 seconds ago) <Tanky Woo>
* 2f1e828 - (origin/test, origin/master, origin/HEAD, master) update test-git-submodule (2 days ago) <Tanky Woo>
如果做 no fast forward 合并:
TankyWoo@Mac::test-git/ (master) » git merge -no-ff ff
查看log:
* a368dcc - (HEAD, master) Merge branch 'ff' (4 seconds ago) <Tanky Woo>
|\
| * 8bffa7b - (ff) update c (3 minutes ago) <Tanky Woo>
| * 525481f - add c (3 minutes ago) <Tanky Woo>
|/
* 2f1e828 - (origin/test, origin/master, origin/HEAD) update test-git-submodule (2 days ago) <Tanky Woo>
再尝试做一个正常情况下不可 fast forward 的，先在ff分支里做两次commit，然后checkout回master分支，也做一个commit。
ff分支的log:
* 693ad65 - (HEAD, ff) update c (12 seconds ago) <Tanky Woo>
* a9ce393 - add c (24 seconds ago) <Tanky Woo>
* 2f1e828 - (origin/test, origin/master, origin/HEAD, master) update test-git-submodule (2 days ago) <Tanky Woo>
master分支的log:
* 503393f - (HEAD, master) add d (1 second ago) <Tanky Woo>
* 2f1e828 - (origin/test, origin/master, origin/HEAD) update test-git-submodule (2 days ago) <Tanky Woo>
可以看到commit id为2f1e828 的提交是master和ff的公共原点，也就是ff是在这个commit上从master分支checkout出去的。
如果直接做-ff-only，可以看到报错提示:
TankyWoo@Mac::test-git/ (master) » git merge --ff-only ff
fatal: Not possible to fast-forward, aborting.
这个时候，要么做一次--no-ff提交合并，要么就rebase，如:
TankyWoo@Mac::test-git/ (master) » git rebase master
做完rebase后的log:
* 3b2a4d9 - (HEAD, master) add d (77 seconds ago) <Tanky Woo>
* 693ad65 - (ff) update c (15 minutes ago) <Tanky Woo>
* a9ce393 - add c (15 minutes ago) <Tanky Woo>
* 2f1e828 - (origin/test, origin/master, origin/HEAD) update test-git-submodule (2 days ago) <Tanky Woo>