Oops! An exception has been thrown during the rendering of a template ("Parameter "branch" for route "branch" must match "[\w-._\/]+" ("Amélioration_du_js" given).") in "tree.twig" at line 3.
Are you using an older version? A change that fixes this (among other things) landed in master recently.
pattern \w matches any alpha numeric character including underscore _.
so, the problem not in underscore. it looks like problem is in special char é in branch name.
I have fixed this in 4eaebed.