Let git-hyper-blame ignore format commit.

Problem:  The 'Reformat WebRTC code base' commit pollutes blame history.
Solution: Add it in ignore list.

Caveats: (from man git-hyper-blame)
  When a line skips over an ignored commit, a guess is made as to which
  commit previously modified that line, but it is not always clear where
  the line came from.
  If the ignored commit makes lots of changes in close proximity,
  in particular adding/removing/reordering lines, then the wrong
  authors may be blamed for nearby edits.

Bug: webrtc:9340
Change-Id: I941805ad6eaaf7bc21cc372b8224784293390d33
Reviewed-on: https://webrtc-review.googlesource.com/84180
Reviewed-by: Yves Gerey <yvesg@webrtc.org>
Reviewed-by: Patrik Höglund <phoglund@webrtc.org>
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23679}

.git-blame-ignore-revs

@@ -18,3 +18,5 @@
 
 # Format all Java in WebRTC.
 b6760f9e4442410f2bcb6090b3b89bf709e2fce2
+# Format all C++ in WebRTC.
+665174fdbb4e0540eccb27cf7412348f1b65534c