Allow fetch.prune and remote.*.prune configuration variables to be set,
and "git fetch" to behave as if "--prune" is given.
"git fetch" that honors remote.*.prune is fine, but I wonder if we
should somehow make "git push" aware of it as well. Perhaps
remote.*.prune should not be just a boolean, but a 4-way "none",
"push", "fetch", "both"?
* ms/fetch-prune-configuration:
fetch: make --prune configurable