Stablise program name in generated code preamble (#610)

This PR fixes inconsistencies when running `protoc-gen-connect-go` on
Windows vs Linux.

cmd/protoc-gen-connect-go/main.go

@@ -150,7 +150,13 @@ func generatePreamble(g *protogen.GeneratedFile, file *protogen.File) {
 	leadingComments(g, protogen.Comments(syntaxLocation.LeadingComments), false /* deprecated */)
 	g.P()
 
-	g.P("// Code generated by ", filepath.Base(os.Args[0]), ". DO NOT EDIT.")
+	programName := filepath.Base(os.Args[0])
+	// Remove .exe suffix on Windows so that generated code is stable, regardless
+	// of whether it was generated on a Windows machine or not.
+	if ext := filepath.Ext(programName); strings.ToLower(ext) == ".exe" {
+		programName = strings.TrimSuffix(programName, ext)
+	}
+	g.P("// Code generated by ", programName, ". DO NOT EDIT.")
 	g.P("//")
 	if file.Proto.GetOptions().GetDeprecated() {
 		wrapComments(g, file.Desc.Path(), " is a deprecated file.")