git.txt: de-emphasize the implementation detail of a ref

git.txt: de-emphasize the implementation detail of a ref

It is an unimportant implementation detail that branches and tags are
stored somewhere under $GIT_DIR/refs directory, or the name of the commit
that will become the parent of the next commit is stored in $GIT_DIR/HEAD.

What is more important is that branches live in refs/heads and tags live
in refs/tags hierarchy in the ref namespace, and HEAD means the tip of the
current branch.

Signed-off-by: Junio C Hamano <gitster@pobox.com>