Default to fast model first for commit messages (#31385)

I was surprised to see this being done for thread summaries, but not
commit messages.

I believe it's a better default as most people would want a faster
commit message generation without spending premium requests.

Considering how the default fast model for copilot is set to the base
one, this is ideal for me (and likely many others), as opposed to
tweaking the configuration every time the base model changes.

Release Notes:

- git: Default to fast model first if not configured for generating
commit messages

crates/language_model/src/registry.rs

@@ -311,6 +311,7 @@ impl LanguageModelRegistry {
 
         self.commit_message_model
             .clone()
+            .or_else(|| self.default_fast_model.clone())
             .or_else(|| self.default_model.clone())
     }