Replace incorrect information about HEAD

Avoid stating that changing HEAD alters the working tree.  Although that
is a common next step they are not actually the same action and can be
done independently (e.g. `git reset` without `--hard`).

Also state that committing while on a branch only alters the branch,
without actually changing HEAD; HEAD only gets the new commit because it
points to the branch rather than a specific commit.
Merge pull request #77 from aschrab/work_tree

Remove incorrect information about HEAD