Utility func for creating a commitSignKey

[eversC]

I feel like it's worth creating a utility func to enable users to easily create a commitSignKey from armoredKeyRing, key identity and passphrase strings (if applicable).

Something akin to what's in https://github.com/src-d/go-git/blob/master/worktree_commit_test.go (obviously minus all the test assertions):

func commitSignKey(c *C, decrypt bool) *openpgp.Entity {
	s := strings.NewReader(armoredKeyRing)
	es, err := openpgp.ReadArmoredKeyRing(s)
	c.Assert(err, IsNil)

	c.Assert(es, HasLen, 1)
	c.Assert(es[0].Identities, HasLen, 1)
	_, ok := es[0].Identities["foo bar <foo@foo.foo>"]
	c.Assert(ok, Equals, true)

	key := es[0]
	if decrypt {
		err = key.PrivateKey.Decrypt([]byte(keyPassphrase))
		c.Assert(err, IsNil)
	}

	return key
}

I had a look into the code, and Issues, and couldn't find anything related.

What're people's thoughts on this.. Is it worth it? Any downsides? Or have I missed something?