Docs: split up pull-fetch-param.txt

Docs: split up pull-fetch-param.txt

The push and pull man pages include a bunch of shared text from
pull-fetch-param.txt.  This simplifies maintenance somewhat, but
there's actually quite a bit of text that applies only to one or the
other.

So, separate out the push- and pull/fetch-specific text into
pull-fetch-param.txt and git-push.txt, then include the largest chunk
of common stuff (the description of protocols and url's) from
urls.txt.  That cuts some irrelevant stuff from the man pages without
making us duplicate too much.

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