merge: log the manual merged file paths

Summary: it's slow to generate the conflicted file via source, dest and base commits, let's just log them

Reviewed By: sggutier

Differential Revision: D48915787

fbshipit-source-id: fd1e153cb1603513cebf6354cbbb81a924db508b

eden/scm/edenscm/merge.py

@@ -1875,6 +1875,7 @@ def applyupdates(repo, actions, wctx, mctx, overwrite, labels=None, ancestors=No
                     src_hex=_gethex(mctx),
                     repo=reponame,
                     manual_merge_files_count=len(files),
+                    manual_merge_files=",".join(files),
                 )
         finally:
             ms.commit()