feat: support comparing against main on forked repo

When doing some open source work I realized I couldn't easily compare my
forked branch against the upstream repos main branch.

main.go

@@ -21,15 +21,33 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-
 	currentBranch := ref.Name().Short()
 
 	var compareArgument string
+	var forkedRepo bool
+	var originRepo string
+
+	remotes, err := gitRepo.Remotes()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for _, remote := range remotes {
+		if remote.Config().Name == "upstream" {
+			forkedRepo = true
+		}
 
-	if len(os.Args) == 2 {
+		if remote.Config().Name == "origin" {
+			originRepo = parseRepoOriginOwnerFromURL(remote.Config().URLs[0])
+		}
+	}
+
+	if forkedRepo {
+		compareArgument = fmt.Sprintf("main...%s:%s", originRepo, currentBranch)
+	} else if len(os.Args) == 2 {
 		compareArgument = os.Args[1]
 
-		if !(strings.Contains(compareArgument, "..") || strings.Contains(compareArgument, "...")) {
+		if !strings.Contains(compareArgument, "..") && !strings.Contains(compareArgument, "...") {
 			compareArgument = compareArgument + "..." + currentBranch
 		}
 	} else {
@@ -40,7 +58,6 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-
 	url := fmt.Sprintf("https://%s/%s/%s/compare/%s", ghRepo.Host, ghRepo.Owner, ghRepo.Name, compareArgument)
 
 	browser := browser.New("", os.Stdout, os.Stderr)
@@ -49,3 +66,8 @@ func main() {
 		log.Fatal(err)
 	}
 }
+
+func parseRepoOriginOwnerFromURL(url string) string {
+	parts := strings.Split(url, ":")
+	return strings.Split(parts[1], "/")[0]
+}