gc-git-svn: arrange for `git-svn fetch` to run gc-git-svn.sh

gc-git-svn: arrange for `git-svn fetch` to run gc-git-svn.sh

When `git svn fetch` runs `git gc --auto`, arrange for it to
end up running the `jobd/gc-git-svn.sh` script.

This will allow Girocco to perform any needed loose object
packing during a long `git-svn fetch` operation.

Up until now, all garbage collection has been entirely
suppressed during `git-svn fetch` operations potentially
resulting in an excessively large number of loose objects
being created before they were ultimately packed up.

The `gc-git-svn.sh` script currently does nothing but will
be filled in by a future commit.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>