strip: move into core As discussed at the 5.2 sprint, replace strip extension by a core command, debugstrip. Obviously, the extension stays for backwards compatibility. As an implementation note, I moved the strip file as is into core, which is not done elsewhere, AFAIK. I could have inlined it into debugcommands, but that doesn't sound great. Differential Revision: https://phab.mercurial-scm.org/D9285