Add ignore-revs file for git blame (#515)

A small change to add an ignore-revs file for `git blame`. This file
contains a list of revisions that `git blame` will ignore, usually meant
for preserving a useful blame history through mass reformats or similar
bulk changes. You can configure your git to use this ignore-revs file
by running `git config blame.ignoreRevsFile .git-blame-ignore-revs`.

The list of commits in this ignore-revs file was generated via:
`git --no-pager log --grep="Format Rust code using rustfmt" --author="github-actions" --format="%H"`

See the following for an explanation and docs:
https://akrabat.com/ignoring-revisions-with-git-blame/
https://www.git-scm.com/docs/git-blame#Documentation/git-blame.txt---ignore-revs-fileltfilegt

.git-blame-ignore-revs

@@ -0,0 +1,33 @@
+# Automated rustfmt commits
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