Pass commit data to ProcessCommitWorker

This changes ProcessCommitWorker so that it takes a Hash containing commit data instead of a commit SHA. This means the worker doesn't have to access Git just to process a commit message (and other data it may use). This in turn should solve the problem of ending up with 15 000-something jobs in the process_commit queue that take forever to process.

app/models/commit.rb

@@ -48,6 +48,10 @@ class Commit
         max_lines: DIFF_HARD_LIMIT_LINES,
       }
     end
+
+    def from_hash(hash, project)
+      new(Gitlab::Git::Commit.new(hash), project)
+    end
   end
   attr_accessor :raw