Workhorse to serve the output for format-patch

Now this is done by Rails which is sad for the Unicorn worker, so this
is moved here. This commit excludes tests for now, which will be added
after the weekend.

internal/git/format-patch.go

+package git
+
+import (
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+
+	"gitlab.com/gitlab-org/gitlab-workhorse/internal/helper"
+	"gitlab.com/gitlab-org/gitlab-workhorse/internal/senddata"
+)
+
+type patch struct{ senddata.Prefix }
+type patchParams struct {
+	RepoPath string
+	ShaFrom  string
+	ShaTo    string
+}
+
+var SendPatch = &diff{"git-format-patch:"}
+
+func (p *patch) Inject(w http.ResponseWriter, r *http.Request, sendData string) {
+	var params patchParams
+	if err := p.Unpack(&params, sendData); err != nil {
+		helper.Fail500(w, fmt.Errorf("SendPatch: unpack sendData: %v", err))
+		return
+	}
+
+	log.Printf("SendPatch: sending patch between %q and %q for %q", params.ShaFrom, params.ShaTo, r.URL.Path)
+
+	gitRange := fmt.Sprintf("%v...%v", params.ShaFrom, params.ShaTo)
+	gitPatchCmd := gitCommand("", "git", "--git-dir="+params.RepoPath, "format-patch", gitRange, "--stdout")
+
+	stdout, err := gitPatchCmd.StdoutPipe()
+	if err != nil {
+		helper.Fail500(w, fmt.Errorf("SendPatch: create stdout pipe: %v", err))
+		return
+	}
+
+	if err := gitPatchCmd.Start(); err != nil {
+		helper.Fail500(w, fmt.Errorf("SendPatch: start %v: %v", gitPatchCmd, err))
+		return
+	}
+	defer helper.CleanUpProcessGroup(gitPatchCmd)
+
+	w.Header().Del("Content-Length")
+	if _, err := io.Copy(w, stdout); err != nil {
+		helper.LogError(fmt.Errorf("SendPatch: copy %v stdout: %v", gitPatchCmd, err))
+		return
+	}
+	if err := gitPatchCmd.Wait(); err != nil {
+		helper.LogError(fmt.Errorf("SendPatch: wait for %v: %v", gitPatchCmd, err))
+		return
+	}
+}

internal/upstream/routes.go

@@ -49,6 +49,7 @@ func (u *Upstream) configureRoutes() {
 		git.SendArchive,
 		git.SendBlob,
 		git.SendDiff,
+		git.SendPatch,
 	)
 
 	u.Routes = []route{