Fixes to commit button in Git Panel (#24425)

Git Panel updates:

* Fixes commit/commit all button to work (and be disabled correctly in
merge conflict status)
* Updates keyboard shortcuts and sets focus on the button (enter now
does the same as click; tab cycles between editor and change list)


Closes #ISSUE

Release Notes:

- N/A *or* Added/Fixed/Improved ...

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/proto/proto/zed.proto

@@ -368,6 +368,7 @@ enum ErrorCode {
     DevServerProjectPathDoesNotExist = 16;
     RemoteUpgradeRequired = 17;
     RateLimitExceeded = 18;
+    CommitFailed = 19;
     reserved 6;
     reserved 14 to 15;
 }