I have accidentally stumbled upon a book with some early drafts of Hugo’s works, and the OFPD scene was there. Roughly translated:
On the first floor they’re about to shoot Enjolras. Grantaire wakes up and says: I am with him. Shoot me.
Enjolras grasps [?] his hand.
'Thank you,' says Gr.
And hold on, just imagine this: Grantaire stands in front of the guns and says “I am with him. Shoot me.” He doesn’t announce his support for the cause, only his allegiance to Enjolras. And it seems that he doesn’t even want to die with him, because he doesn’t feel like he’s worth it. He rather hopes he can die instead of him, or at least before him. As if he was saying “If you want to shoot him, you’ll have to shoot me first.”
Grantaire doesn’t say anything to Enjolras, so he’s probably looking at the soldiers the whole time. And here we have Enjolras, who sees Grantaire, whom he used to scorn so often, consciously choosing to die for him. And acting as if he thought Enjolras would let him be executed right there in front of him. So he has to do something to let him know that he doesn’t really think Grantaire is good for nothing, that he accepts him, that he’s grateful. So he grasps his hand. And Grantaire, surprised, finally looks him in the eyes, and understands everything. And he says: “Thank you.”