Git的基本概念

HEAD

HEAD对应当前的状态下的最后一次提交。HEAD对应索引，而非分支。同理FETCH_HEAD.

FETCH_HEAD

FETCH_HEAD is a short-lived ref, to keep track of what has just been fetched from the remote repository. git pull first invokes git fetch, in normal cases fetching a branch from the remote; FETCH_HEAD points to the tip of this branch (it stores the
SHA1 of the commit, just as branches do). git pull then invokes git merge, merging FETCH_HEAD into the current branch.

The result is exactly what you'd expect: the commit at the tip of the appropriate remote branch is merged into the commit at the tip of your current branch.

This is a bit like doing git fetch without arguments (or git remote update), updating all your remote branches, then running git merge origin/<branch>, but using FETCH_HEAD internally instead to refer to whatever single ref was fetched, instead of needing to
name things.

参考：http://stackoverflow.com/questions/9237348/what-does-fetch-head-in-git-mean