version: construct short hash in dev mode if GitCommit is given.

Allows a dev built to provide GitCommit and have the short hash
computed correctly, even if the Go embedded build info lacks a
git commit.

Signed-off-by: David Anderson <dave@natulte.net>

version/version.go

@@ -41,14 +41,11 @@ func init() {
 		return
 	}
 	var dirty string // "-dirty" suffix if dirty
-	var commitHashAbbrev, commitDate string
+	var commitDate string
 	for _, s := range bi.Settings {
 		switch s.Key {
 		case "vcs.revision":
 			GitCommit = s.Value
-			if len(s.Value) >= 9 {
-				commitHashAbbrev = s.Value[:9]
-			}
 		case "vcs.time":
 			if len(s.Value) >= len("yyyy-mm-dd") {
 				commitDate = s.Value[:len("yyyy-mm-dd")]
@@ -61,6 +58,10 @@ func init() {
 			}
 		}
 	}
+	commitHashAbbrev := GitCommit
+	if len(commitHashAbbrev) >= 9 {
+		commitHashAbbrev = commitHashAbbrev[:9]
+	}
 
 	// Backup path, using Go 1.18's built-in git stamping.
 	Short = strings.TrimSpace(tailscaleroot.Version) + "-dev" + commitDate