You see the Git documentation saying things like
The branch must be fully merged in HEAD.
But what is Git HEAD
exac
Take a look at Creating and playing with branches
HEAD is actually a file whose contents determines where the HEAD variable refers:
$ cat .git/HEAD
ref: refs/heads/master
$ cat .git/refs/heads/master
35ede5c916f88d8ba5a9dd6afd69fcaf773f70ed
In this repository, the contents of the HEAD file refers to a second file named refs/heads/master. The file refs/heads/master contains the hash of the most recent commit on the master branch.
The result is HEAD points to the master branch commit from the .git/refs/heads/master file.