dirstate: perform transactions with _copymap using single call, where possible This replaces patterns such as this: ``` if f in self._copymap: del self._copymap[f] ``` with this: ``` self._copymap.pop(f, None) ``` Although eliminating the extra lookup/call may be a negligible performance win in the standard dirstate, alternative implementations, such as [sqldirstate](https://bitbucket.org/facebook/hg-experimental/src/default/sqldirstate/) may see a bigger win where each of these calls results in an RPC, so the savings is greater. Test Plan: `make tests` Differential Revision: https://phab.mercurial-scm.org/D493