subtree: fix argument validation in add/pull/push

When working with a remote repository add/pull/push do not accept a
<refspec> as parameter but just a <ref>.

This patch:
 - relaxes the check the <ref> argument in "git subtree add <repo>"
   (previous code would not accept a ref name that does not exist
   locally too, new code only ensures that the ref is well formatted)

 - add the same check in "git subtree pull/push" + check the number of
   parameters

 - update the doc to use <ref> instead of <refspec>

Signed-off-by: Anthony Baire <Anthony.Baire@irisa.fr>