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]
Although eliminating the extra lookup/call may be a negligible performance win
in the standard dirstate, alternative implementations, such as
may see a bigger win where each of these calls results in an RPC,
so the savings is greater.
Differential Revision: https://phab.mercurial-scm.org/D493