merge: drop legacy distance calculation

Summary:
The legacy distance calculation prevents large O(N) scans by using an approx.

With segmented changelog, distance calculation is no longer O(N) and the approx
can be far off if two commits belong to different groups (ie. one in "master"
one in "non-master") - they will have (1 << 56) distance.

Reviewed By: zzl0

Differential Revision: D55436320

fbshipit-source-id: eb3e5bedf7df1e629c285c712a844da7bc8a2521

eden/scm/sapling/merge.py

@@ -1853,15 +1853,7 @@ def _logupdatedistance(ui, repo, node):
         # doesn't play nicely with revsets later because it resolve to the tip
         # commit.
         node = repo[node].node()
-        revdistance = abs(repo["."].rev() - repo[node].rev())
-        if revdistance == 0:
-            distance = 0
-        elif revdistance >= 100000:
-            # Calculating real distance is too slow.
-            # Use an approximate.
-            distance = ((revdistance + 500) / 1000) * 1000
-        else:
-            distance = len(repo.revs("(%n %% .) + (. %% %n)", node, node))
+        distance = len(repo.revs("(%n %% .) + (. %% %n)", node, node))
         repo.ui.log("update_size", update_distance=distance)
     except Exception:
         # error may happen like: RepoLookupError: unknown revision '-1'