I'm thinking it might be Partridge Pea. The last part of this blog has some that looks just like it. I am not finding much online about tortoises and it though...
http://thepapershell.com/2013/07/
I think you're right, Dawn. Vetch's fronds are further apart and Mimosa seedlings look slightly different too:
View attachment 134084
The blog you showed says it's good for forage, so if you don't mind it growing in your yard, it's edible.