chore: add a Git .mailmap with my new name (#10926)

mgol · andrewseguin · commit 7878d235f8c2 · 2019-01-11T14:37:55.000-08:00
In this way my past contribution is mapped correctly.
diff --git a/.mailmap b/.mailmap
@@ -0,0 +1 @@
+Michał Gołębiowski-Owczarek <m.goleb@gmail.com>

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	`+Michał Gołębiowski-Owczarek <[email protected]>`