[#6030] Set context on commits during compute_diffs to avoid calling guess_repo

Signed-off-by: Cory Johns <cjohns@slashdotmedia.com>
diff --git a/Allura/allura/model/repo_refresh.py b/Allura/allura/model/repo_refresh.py
index 539df2a..2e176af 100644
--- a/Allura/allura/model/repo_refresh.py
+++ b/Allura/allura/model/repo_refresh.py
@@ -94,7 +94,9 @@
     # with caching.
     if repo._refresh_precompute:
         for i, oid in enumerate(commit_ids):
-            ci = CommitDoc.m.find(dict(_id=oid), validate=False).next()
+            cid = CommitDoc.m.find(dict(_id=oid), validate=False).next()
+            ci = mapper(Commit).create(cid, dict(instrument=False))
+            ci.set_context(repo)
             compute_diffs(repo._id, cache, ci)
             if (i+1) % 100 == 0:
                 log.info('Compute diffs %d: %s', (i+1), ci._id)