I think we shouldn't use rebase for branches

Since ACE is mostly included to projects as submodules, I think it's not reasonable since old commits may have broken references to proper submodule commits.

I've linked ACE's submodule `ptplayer` branch in the past as I was working on it while developing Germz. Now that I've repeatedly rebased it, submodule references are broken in old commits.

I think it's better to just merge branches with master to sync them up. This way, it's less destructive for projects which use experimental / unstable features.