[FIX] runbot_merge: maintenance gc command

`gc --prune` can not take a *separate* parameter, it has to be part of
the same arg (the `=` is not optional), otherwise the `gc` call blows
up.

So use the positional form of the git command to generate the correct
invocation, Python-level `foo=bar` generates a split-style option in
two args which does not please git.

runbot_merge/models/crons/git_maintenance.py

@@ -32,6 +32,6 @@ class GC(models.TransientModel):
             r = repo_git\
                 .stdout(True)\
                 .with_config(stderr=subprocess.STDOUT, text=True, check=False)\
-                .gc(aggressive=True, prune='now')
+                .gc('--prune=now', aggressive=True)
             if r.returncode:
                 _gc.warning("Maintenance failure (status=%d):\n%s", r.returncode, r.stdout)