help: pass commands module by argument This removes import cycle.

copies: introduce getdstfctx Previously `c2` may had an incorrect linkrev because getsrcfctx set wrong _descendantrev. getsrcfctx() sets descendant rev equals to srcctx.rev() (see _makegetfctx()), but for `c2` descendant rev should be dstctx. While we were lucky it didn't broke copytracing it made it significantly slower in some cases. Besides it broke some external extensions, for example remotefilelog.

copies: rename getfctx to getsrcfctx In the next patch we'll use getdstfctx. Let's rename getfctx to getsrcfctx in this patch.