func (s *RepositoriesService) GetCommitSHA1(owner, repo, ref, lastSHA string) (string, *Response, error) {
	u := fmt.Sprintf("repos/%v/%v/commits/%v", owner, repo, ref)
	req, err := s.client.NewRequest("GET", u, nil)
	if err != nil {
		return "", nil, err
	}
	if lastSHA != "" {
		req.Header.Set("If-None-Match", `"`+lastSHA+`"`)
	}
	req.Header.Set("Accept", mediaTypeV3SHA)
	var buf bytes.Buffer
	resp, err := s.client.Do(req, &buf)
	if err != nil {
		return "", resp, err
	}
	return buf.String(), resp, err
}
GetCommitSHA1 gets the SHA-1 of a commit reference.  If a last-known SHA1 is
supplied and no new commits have occurred, a 304 Unmodified response is returned.

GitHub API docs: https://developer.github.com/v3/repos/commits/#get-the-sha-1-of-a-commit-reference
package github
import (
	"bytes"
	"fmt"
	"time"
)
type RepositoryCommit struct {
	SHA         *string  `json:"sha,omitempty"`
	Commit      *Commit  `json:"commit,omitempty"`
	Author      *User    `json:"author,omitempty"`
	Committer   *User    `json:"committer,omitempty"`
	Parents     []Commit `json:"parents,omitempty"`
	HTMLURL     *string  `json:"html_url,omitempty"`
	URL         *string  `json:"url,omitempty"`
	CommentsURL *string  `json:"comments_url,omitempty"`
	Stats *CommitStats `json:"stats,omitempty"`
	Files []CommitFile `json:"files,omitempty"`
}
func (r RepositoryCommit) String() string {
	return Stringify(r)
}
type CommitStats struct {
	Additions *int `json:"additions,omitempty"`
	Deletions *int `json:"deletions,omitempty"`
	Total     *int `json:"total,omitempty"`
}
func (c CommitStats) String() string {
	return Stringify(c)
}
type CommitFile struct {
	SHA         *string `json:"sha,omitempty"`
	Filename    *string `json:"filename,omitempty"`
	Additions   *int    `json:"additions,omitempty"`
	Deletions   *int    `json:"deletions,omitempty"`
	Changes     *int    `json:"changes,omitempty"`
	Status      *string `json:"status,omitempty"`
	Patch       *string `json:"patch,omitempty"`
	BlobURL     *string `json:"blob_url,omitempty"`
	RawURL      *string `json:"raw_url,omitempty"`
	ContentsURL *string `json:"contents_url,omitempty"`
}
func (c CommitFile) String() string {
	return Stringify(c)
}
type CommitsComparison struct {
	BaseCommit      *RepositoryCommit `json:"base_commit,omitempty"`
	MergeBaseCommit *RepositoryCommit `json:"merge_base_commit,omitempty"`
	Status       *string `json:"status,omitempty"`
	AheadBy      *int    `json:"ahead_by,omitempty"`
	BehindBy     *int    `json:"behind_by,omitempty"`
	TotalCommits *int    `json:"total_commits,omitempty"`
	Commits []RepositoryCommit `json:"commits,omitempty"`
	Files []CommitFile `json:"files,omitempty"`
}
func (c CommitsComparison) String() string {
	return Stringify(c)
}
type CommitsListOptions struct {
	SHA string `url:"sha,omitempty"`
	Path string `url:"path,omitempty"`
	Author string `url:"author,omitempty"`
	Since time.Time `url:"since,omitempty"`
	Until time.Time `url:"until,omitempty"`
	ListOptions
}
func (s *RepositoriesService) ListCommits(owner, repo string, opt *CommitsListOptions) ([]*RepositoryCommit, *Response, error) {
	u := fmt.Sprintf("repos/%v/%v/commits", owner, repo)
	u, err := addOptions(u, opt)
	if err != nil {
		return nil, nil, err
	}
	req, err := s.client.NewRequest("GET", u, nil)
	if err != nil {
		return nil, nil, err
	}
	var commits []*RepositoryCommit
	resp, err := s.client.Do(req, &commits)
	if err != nil {
		return nil, resp, err
	}
	return commits, resp, nil
}
func (s *RepositoriesService) GetCommit(owner, repo, sha string) (*RepositoryCommit, *Response, error) {
	u := fmt.Sprintf("repos/%v/%v/commits/%v", owner, repo, sha)
	req, err := s.client.NewRequest("GET", u, nil)
	if err != nil {
		return nil, nil, err
	}
	req.Header.Set("Accept", mediaTypeGitSigningPreview)
	commit := new(RepositoryCommit)
	resp, err := s.client.Do(req, commit)
	if err != nil {
		return nil, resp, err
	}
	return commit, resp, err
}