add support for git alternates

[taralx]

Simple test to retrieve an object from a lightweight clone results in "object not found" errors.

[taralx]

Steps to reproduce:

package main

import git "gopkg.in/src-d/go-git.v4"

func main() {
        r, _ := git.PlainOpen("test2")
        h, _ := r.Head()
        _, err := r.CommitObject(h.Hash())
        if err != nil {
                panic(err)
        }
}

mkdir test; cd test
git init
git commit -m "test" --allow-empty
cd ..
git clone -s test test2
cd test2
git cat-file -p HEAD
cd ..
go run t.go

Expected result: Successful run.
Actual result: panic: object not found

[smola]

@taralx Thank you for taking the time to report this issue.

We have no immediate plans to implement alternates, but we'll be very happy to accept PRs adding support.