Reduce number of FindCommit calls from RefsController#logs_tree

Fixes gitaly#1057

The old code was calling LastCommitForPath to extract a commit ID _then_
call FindCommit to get a commit it already had in the first place!

app/models/repository.rb

@@ -651,14 +651,15 @@ class Repository
   end
   def last_commit_for_path(sha, path)
-    commit_by(oid: last_commit_id_for_path(sha, path))
+    commit = raw_repository.last_commit_for_path(sha, path)
+    ::Commit.new(commit, @project) if commit
   end
   def last_commit_id_for_path(sha, path)
     key = path.blank? ? "last_commit_id_for_path:#{sha}" : "last_commit_id_for_path:#{sha}:#{Digest::SHA1.hexdigest(path)}"
     cache.fetch(key) do
-      raw_repository.last_commit_id_for_path(sha, path)
+      last_commit_for_path(sha, path)&.id
     end
   end

lib/gitlab/git/repository.rb

@@ -1443,12 +1443,12 @@ module Gitlab
         end
       end
-      def last_commit_id_for_path(sha, path)
+      def last_commit_for_path(sha, path)
         gitaly_migrate(:last_commit_for_path) do |is_enabled|
           if is_enabled
-            last_commit_for_path_by_gitaly(sha, path).id
+            last_commit_for_path_by_gitaly(sha, path)
           else
-            last_commit_id_for_path_by_shelling_out(sha, path)
+            last_commit_for_path_by_rugged(sha, path)
           end
         end
       end
@@ -1896,7 +1896,7 @@ module Gitlab
       end
       def last_commit_for_path_by_rugged(sha, path)
-        sha = last_commit_id_for_path(sha, path)
+        sha = last_commit_id_for_path_by_shelling_out(sha, path)
         commit(sha)
       end