$ git branch foo <2>
$ git tag foo <3>
------------
-
<1> creates a new branch `foo`, which refers to commit `f`, and then
updates `HEAD` to refer to branch `foo`. In other words, we'll no longer
be in detached `HEAD` state after this command.
-
<2> similarly creates a new branch `foo`, which refers to commit `f`,
but leaves `HEAD` detached.
-
<3> creates a new tag `foo`, which refers to commit `f`,
leaving `HEAD` detached.
$ rm -f hello.c
$ git checkout hello.c <3>
------------
-+
<1> switch branch
<2> take a file out of another commit
<3> restore `hello.c` from the index