Add clemensh->clemensb to .mailmap

This adds a .mailmap entry to remap clemensh@chromium.org to
clemensb@chromium.org, with Clemens' new name.

R=mathias@chromium.org

Change-Id: If234c59cf9089e71a1baf56115d5ee900f8af275
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2919626Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75017}

.mailmap

@@ -9,3 +9,5 @@
 # See https://git-scm.com/docs/gitmailmap for more information.
 #
 # Please keep the list sorted.
+#
+Clemens Backes <clemensb@chromium.org> Clemens Hammacher <clemensh@chromium.org>