maintain (generator): simplify build command

Before this commit our build had this option:
-ldflags "-X github.com/mickael-kerjean/filestash/server/common.BUILD_DATE=`date -u +%Y%m%d` -X github.com/mickael-kerjean/filestash/server/common.BUILD_REF=`git rev-parse HEAD`"

Doing this via a generator is much easier for people getting started
with Filestash

server/common/constants.go

@@ -5,6 +5,7 @@ import (
 	"path/filepath"
 )
 
+//go:generate go run ../generator/constants.go
 const (
 	APP_VERSION       = "v0.5"
 	LOG_PATH          = "data/state/log/"

server/generator/constants.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+	"time"
+)
+
+func main() {
+	cmd, b := exec.Command("git", "rev-parse", "HEAD"), new(strings.Builder)
+	cmd.Stdout = b
+	cmd.Run()
+
+	content := fmt.Sprintf(`package common
+
+func init() {
+    BUILD_REF = "%s"
+    BUILD_DATE = "%s"
+}
+`, strings.TrimSpace(b.String()), time.Now().Format("20060102"))
+
+	f, err := os.OpenFile("../common/constants_generated.go", os.O_CREATE|os.O_WRONLY, os.ModePerm)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "error: %v\n", err)
+		os.Exit(1)
+		return
+	}
+	f.Write([]byte(content))
+	f.Close()
+}