Docs: move git url and remotes text to separate sections

Docs: move git url and remotes text to separate sections

The sections on git urls and remotes files in the git-fetch,
git-pull, and git-push manpages seem long enough to be worth a
manpage section of their own.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>