[IMP] runbot_merge: pass commit-tree message via stdin

Automating via parameters is riskier as we can hit the CLI
limitations (cf 0a839a4857). Going
through stdin is a lot safer and cleaner when automating, and it's not
much of an imposition here.

runbot_merge/git.py

@@ -222,6 +222,7 @@ class Repo:
                 authorship['GIT_COMMITTER_DATE'] = committer[2]
 
         return self.with_config(
+            input=message,
             stdout=subprocess.PIPE,
             text=True,
             env={
@@ -235,7 +236,7 @@ class Repo:
         )._run(
             'commit-tree',
             tree,
-            '-m', message,
+            '-F', '-',
             *itertools.chain.from_iterable(('-p', p) for p in parents),
         )