It is not "A la mort", but "Pour la France ! En avant !" (For France ! Forward!). It is mentioned for instance in the letter of military doctor Maurice Antoine Martin-Laval written during WW1:

At that moment the signal rocket was fired by the commander of the attack. Immediately the lieutenants rushed out of the trench shouting "Fix bayonets", "Forward", "Attack", "For France".

We can find it during the Napoleonic wars too (Paulin, 1895):

In the midst of this deluge of fire, [our troops] retained all the national gaiety. A soldier whom his comrades called the Emperor became impatient with the obstinacy of the Prussians: "To me, grenadiers! Forward," he cried; "let's go, follow the Emperor!" he threw himself into the thick of the fray: the troops followed him and the guards were driven in. He was made corporal.

Since it is pretty basic, there are probably older examples. The battle cry of the city of Lyon (sometimes found on its coat of arms) was once (in Provencal) "Avant, avant, Lion le Mehlor" (Forward, Forward, Lyon is the best), identified in a municipal cartulary of 1273.

• Guéno, Jean-Pierre, and Yves Laplume. Paroles de Poilus: lettres et carnets du front, 1914-1918. Librio, 2007.

• Paulin, Jules-Antoine. Les Souvenirs du Général Baron Paulin (1782-1876). Plon, Nourrit & Cie, 1895.