Add a target in GN that generates a last_change.h file for the SVN revision.

tools/gn/command_help.cc

Index: tools/gn/command_help.cc
diff --git a/tools/gn/command_help.cc b/tools/gn/command_help.cc
index 954db121330e9665eaa9ea18be3274ba7aa732fa..8f8611eaa7004af54ebc9d886945253e59060ac2 100644
--- a/tools/gn/command_help.cc
+++ b/tools/gn/command_help.cc
@@ -51,6 +51,8 @@ void PrintToplevelHelp() {
       "--tracelog: Writes a Chrome-compatible trace log to the given file.");
   PrintShortHelp(
       "-v: Verbose mode, print lots of logging.");
+  PrintShortHelp(
+      "--version: Print the GN binary's version and exit.");
 
   // Functions.
   OutputString("\nBuildfile functions (type \"gn help <function>\" for more "