.git-blame-ignore-revs: Add file to handle mass reformatting

Google's git-hyper-blame command is a variant of git-blame,
but it can ignore certain commits like mass reformatting in f53df7d .
To use git-hyper-blame, it is better to make a .git-blame-ignore-revs file
in the repository root directory and put commits in the file to ignore such
commits.

Ref: https://commondatastorage.googleapis.com/chrome-infra-docs/flat/depot_tools/docs/html/git-hyper-blame.html
https://chromium.googlesource.com/chromium/src.git/+/437b870/.git-blame-ignore-revs
https://github.com/mozilla/gecko-dev/commits/03c48ac/.git-blame-ignore-revs

.git-blame-ignore-revs

@@ -0,0 +1 @@
+f53df7da64d2dfc542c24656720b2f47c8957164