keys is not consistent with git-fs-db@0.2.0

[bryanburgers]

The result of the keys method with a prefix is different than the result of the keys method with a prefix for git-fs-db version 0.2.0. See output from the following bit of code:

var platform = require('git-node-platform');
var memdb = require('git-memdb');
var fsDb = require('git-fs-db')(platform);
var fs = platform.fs;

var filesystemDb = fsDb(fs('tmp'));
var memoryDb = memdb();

function doit(db, callback) {
    db.set('refs/heads/master', 'a874178823acbf493d12f44fc6aac5b3869903f1', function() {
        db.set('refs/heads/old', 'a874178823acbf493d12f44fc6aac5b3869903f1', function() {
            db.set('refs/tags/v1.0.0', 'a874178823acbf493d12f44fc6aac5b3869903f1', function() {
                db.keys('refs', function(err, list) {
                    callback(null, list);
                });
            });
        });
    });
}

memoryDb.init(function() {
    doit(filesystemDb, function(err, list) {
        console.log(list);
        doit(memoryDb, function(err, list) {
            console.log(list);
        });
    });
});

Outputs:

[ 'heads', 'tags' ]
[ '/heads/master', '/heads/old', '/tags/v1.0.0' ]

So while git-fs-db normalizes the output from keys, git-memdb does not.

This causes js-git@0.5.4#fetch to fetch more than it needs to.