dwim shorthand refspecs for fetch

`git` allows you to store the sloppy dwim-needing refspecs in the configuration file, so we need to be able to use them inside the library, as we load them in `git_remote_load`.

While I still don't like implementing these dwim rules inside the library, this is better than the alternative wrt loading them from config.

[There are similar rules for push I'd like to get in at some point when I have time and can figure out how to test it]

This fixes #1337.