git remote: Separate usage strings for subcommands

git remote: Separate usage strings for subcommands

When the usage string for a subcommand must be printed,
only print the information relevant to that command.

This commit also removes the complete options list from
the first line of the subcommand usage string. Instead,
individual options are documented in the detailed
description following the general usage line.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>