glossary: Update and rephrase the definition of a remote-tracking branch
The definition of a remote-tracking branch in the glossary have been
out-of-date for a while (by e.g. referring to "Pull:" from old-style
$GIT_DIR/remotes files).
Also, the preceding patches have formalized that a remote-tracking branch
must match a configured refspec in order to be usable as an upstream.
This patch rewrites the paragraph on remote-tracking branches accordingly.
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>