Add a .git-blame-ignore-rev file (#3277)

`git blame` can be configured to ignore certain commits, such as
formatting commits. This patch adds a `.git-blame-ignore-revs` file (using
a common name) with a few reformatting commits that can be used with
`git blame --ignore-revs-file .git-blame-ignore-revs` or configured as a
default ignore file with:

```
$ git config blame.ignoreRevsFile .git-blame-ignore-revs
```

Hopefully we should rarely need to add new commits to this file (except
on .ocamlformat changes) thanks to better formatting discipline, but
331c167 (Reformat middle_end/flambda2/) is making `git blame`
effectively useless for archeological tasks. I also heuristically added
a few other commits with "Reformat" in the name, but I have not seen
them actually show up as problematic.

Author: bclement-ocp

.git-blame-ignore-revs

@@ -0,0 +1,8 @@
+# Reformat middle_end/flambda2/ (#230)
+331c16734636a218261d4835fb77b38c5788f50a
+
+# Reformat after #2142 (#2149)
+6d21416746f4d712a164359d706cf00aae539f93
+
+# Reformat to cope with .ocamlformat change
+9777982cfb3733cd21896797b0b21980f5473462