Add bors to .mailmap.

nnethercote · nnethercote · commit b2e05f6400ca · 2022-11-30T13:09:47.000+11:00
Before:
```
$ git shortlog -se | grep "\&lt;bors\&gt;"
 27755	bors &lt;bors@rust-lang.org&gt;
  5729	bors[bot] &lt;26634292+bors[bot]@users.noreply.github.com&gt;
   995	bors[bot] &lt;bors[bot]@users.noreply.github.com&gt;
```
After:
```
$ git shortlog -se | grep "\&lt;bors\&gt;"
 34479	bors &lt;bors@rust-lang.org&gt;
```
diff --git a/.mailmap b/.mailmap
@@ -73,6 +73,8 @@ Björn Steinbrink <bsteinbr@gmail.com> <B.Steinbrink@gmx.de>
 blake2-ppc <ulrik.sverdrup@gmail.com> <blake2-ppc>
 boolean_coercion <booleancoercion@gmail.com>
 Boris Egorov <jightuse@gmail.com> <egorov@linux.com>
+bors <bors@rust-lang.org> bors[bot] <26634292+bors[bot]@users.noreply.github.com>
+bors <bors@rust-lang.org> bors[bot] <bors[bot]@users.noreply.github.com>
 Braden Nelson <moonheart08@users.noreply.github.com>
 Brandon Sanderson <singingboyo@gmail.com> Brandon Sanderson <singingboyo@hotmail.com>
 Brett Cannon <brett@python.org> Brett Cannon <brettcannon@users.noreply.github.com>

-Original file line number
+Diff line change
 blake2-ppc <[email protected]> <blake2-ppc>
 boolean_coercion <[email protected]>
 Boris Egorov <[email protected]> <[email protected]>
 +bors <[email protected]> bors[bot] <26634292+bors[bot]@users.noreply.github.com>
 +bors <[email protected]> bors[bot] <bors[bot]@users.noreply.github.com>
 Braden Nelson <[email protected]>
 Brandon Sanderson <[email protected]> Brandon Sanderson <[email protected]>
 Brett Cannon <[email protected]> Brett Cannon <[email protected]>