Add fatal missing-version handling with opt-out flag

src/git.go

@@ -86,7 +86,12 @@ func gitCommit(bc *BumpConfig, newVersion string, configPath string) {
 			log.Fatalf("Stat error for %s: %v", p, err)
 		}
 
-		st := status[p]
+		st, ok := status[p]
+		if !ok || st == nil {
+			// nothing to stage for this file
+			continue
+		}
+
 		if st.Worktree != git.Unmodified {
 			if _, err := worktree.Add(p); err != nil {
 				log.Fatalf("Add %s error: %v", p, err)