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
== New Features ==
* `hg strip`, from the strip extension, is now a core command, `hg
debugstrip`. The extension remains for compatibility.
== New Experimental Features ==
== Bug Fixes ==
== Backwards Compatibility Changes ==
== Internal API Changes ==