contrib/subtree: Document pull-all and push-all

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>
contrib/subtree: Document from-submodule and prune commands

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>
contrib/subtree: Add missing commands to SYNOPSIS

Add pull-all, push-all, from-submodule, prune and diff commands
to SYNOPSIS.

Add alternate parameter option for add command.

Use long-form option for specifying prefix.

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>

Conflicts:
	contrib/subtree/git-subtree.txt
contrib/subtree: Document list command

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>

Conflicts:
	git-subtree.sh
contrib/subtree: Fix order of case switches so default case is last

Signed-off-by: Paul Campbell <pcampbell@kemitix.net>