remote: verify there is a push foreach callback set
remote: add test for pushing and deleting with the local transport
local: adjust to the common refspec parser

We no longer have NULL strings, but empty ones and duplicate the sides
if necessar, so the first check will never do anything.

While in the area, remove unnecessary ifs and early returns.
Merge pull request #2730 from libgit2/cmn/local-push

Adjust the local transport for the common refspec parser