Teach Git to ignore the tools/extra directory.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161764 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/.gitignore b/.gitignore
index ddd6638..b906ab9 100644
--- a/.gitignore
+++ b/.gitignore
@@ -23,3 +23,9 @@
 #==============================================================================#
 cscope.files
 cscope.out
+
+#==============================================================================#
+# Directories to ignore (do not add trailing '/'s, they skip symlinks).
+#==============================================================================#
+# Clang extra user tools, which is tracked independently (clang-tools-extra).
+tools/extra