Documentation: clone: clarify discussion of initial branch

Documentation: clone: clarify discussion of initial branch

When saying the initial branch is equal to the currently active
remote branch, it is probably intended that the branch heads
point to the same commit.  Maybe it would be more useful to a
new user to emphasize that the tree contents and history are the
same.

More important, probably, is that this new branch is set up so
that "git pull" merges changes from the corresponding remote
branch.  The next paragraph addresses that directly.  What the
reader needs to know to begin with is that (1) the initial branch
is your own; if you do not pull, it won't get updated, and that
(2) the initial branch starts out at the same commit as the
upstream.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>