This code should just call sha1.New(). If for some reason these allocations are a performance issue it is very simple to fix that will a little pooling:

    package main

    import (
        "crypto/sha1"
        "hash"
        "sync"
    )

    func main() {
        hasher := getHasher()
        defer poolHasher(hasher)
    }

    var hasherPool sync.Pool = sync.Pool{
        New: func() interface{} {
            return sha1.New()
        },
    }

    func getHasher() hash.Hash {
        return hasherPool.Get().(hash.Hash)
    }

    func poolHasher(hasher hash.Hash) {
        hasher.Reset()
        hasherPool.Put(hasher)
    }