user-manual: introduce "branch" and "branch head" differently

user-manual: introduce "branch" and "branch head" differently

I was using "branch" to mean "head", but that's perhaps a little
sloppy; so instead start by using the terms "branch head" and "head",
while still quickly falling back on "branch", since that's what
people actually say more frequently.

Also include glossary references on the first uses of "head" and "tag".

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>