Allow the Bug prefix fxb/ to be used to Fuchsia bug numbers
in commit messages. These should be redirected to
https://fxbug.dev/<number>

Bug: None
Change-Id: Ibdba2b22b15cc6851dccc60e64bea034b8be668a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/44129
Presubmit-Ready: David Turner <digit@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: David Turner <digit@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>