git-llvm script should add .exe on Windows.

llvm-svn: 303708
diff --git a/llvm/utils/git-svn/git-llvm b/llvm/utils/git-svn/git-llvm
index 26f8f4d..975b848 100755
--- a/llvm/utils/git-svn/git-llvm
+++ b/llvm/utils/git-svn/git-llvm
@@ -134,6 +134,8 @@
                  ignore_errors=kwargs.get('ignore_errors', None))
 
 def program_exists(cmd):
+    if sys.platform == 'win32' and not cmd.endswith('.exe'):
+        cmd += '.exe'
     for path in os.environ["PATH"].split(os.pathsep):
         if os.access(os.path.join(path, cmd), os.X_OK):
             return True