Check for SVN earlier in clang's update.py script

tools/clang/scripts/update.py

Index: tools/clang/scripts/update.py
diff --git a/tools/clang/scripts/update.py b/tools/clang/scripts/update.py
index 3953d672946bb17cc522ae7d2f8eb63d52627682..2ea0d6d3d21b8651672061f4e6df6abc2cd3ae42 100755
--- a/tools/clang/scripts/update.py
+++ b/tools/clang/scripts/update.py
@@ -450,7 +450,6 @@ def UpdateClang(args):
     return 1
 
   DownloadHostGcc(args)
-  AddSvnToPathOnWin()
   AddCMakeToPath()
   AddGnuWinToPath()
 
@@ -881,6 +880,11 @@ def main():
       print 'Skipping Clang update (make_clang_dir= was set in GYP_DEFINES).'
       return 0
 
+  # Get svn if we're going to use it to check the revision or do a local build.
+  if (use_head_revision or args.llvm_force_head_revision or
+      args.force_local_build):
+    AddSvnToPathOnWin()
+
   global CLANG_REVISION, PACKAGE_VERSION
   if args.print_revision:
     if use_head_revision or args.llvm_force_head_revision: