add .git-blame-ignore-revs

This is for https://gn-review.git.corp.google.com/c/gn/+/16440.

Change-Id: I5fe731ce9c06cca6966ffdf77cc275826e292391
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/16500
Commit-Queue: Takuto Ikuta <tikuta@google.com>
Reviewed-by: Petr Hosek <phosek@google.com>

.git-blame-ignore-revs

diff --git a/.git-blame-ignore-revs b/.git-blame-ignore-revs
new file mode 100644
index 0000000..3a8eca8
--- /dev/null
+++ b/.git-blame-ignore-revs
@@ -0,0 +1,2 @@
+# apply clang-format to all C++ sources
+8bd36a27c0764c869d40ac4102a24720b781b389