Add a Git .mailmap with mgol's new name

mgol · xzyfer · commit a60caf43fccd · 2018-02-20T19:56:49.000+11:00
In this way his past contributions are mapped correctly, including collapsing
two different ways of writing "ę" into one.
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]>`