clone 代码到本地：
git clone git链接
# Clever @ clever in~/Downloads/common on git:master o [11:33:05]
$ git status
On branch master
Your branch isup-to-date with 'origin/master'.
nothing to commit,working tree clean
# Clever @ clever in~/Downloads/common on git:master o [11:33:16]
$ git checkout dev
Branch dev set up totrack remote branch dev from origin.
Switched to a newbranch 'dev'
取回远程主机某个分支的更新，再与本地的指定分支合并
index暂存区
