merge: show number of ancestors in bid merge debug notes

A good number of times, we don't generate an action for a file in
manifestmerge() which can result in bid merge doing the wrong thing.

Mentioning the number of ancestors from which we are bidding will help spot such
cases where we are not returning an action.

Differential Revision: https://phab.mercurial-scm.org/D8967

mercurial/merge.py

@@ -1151,7 +1151,10 b' def calculateupdates('
 
         # Call for bids
         # Pick the best bid for each file
-        repo.ui.note(_(b'\nauction for merging merge bids\n'))
+        repo.ui.note(
+            _(b'\nauction for merging merge bids (%d ancestors)\n')
+            % len(ancestors)
+        )
         mresult = mergeresult()
         for f, bids in sorted(fbids.items()):
             # bids is a mapping from action method to list af actions

tests/test-merge-criss-cross.t

@@ -149,7 +149,7 b' Redo merge with merge.preferancestor="*"'
    f1: versions differ -> m
    f2: remote unchanged -> k
   
-  auction for merging merge bids
+  auction for merging merge bids (2 ancestors)
    f1: picking 'get' action
    f2: picking 'keep' action
   end of auction
@@ -192,7 +192,7 b' The other way around:'
    f1: versions differ -> m
    f2: remote is newer -> g
   
-  auction for merging merge bids
+  auction for merging merge bids (2 ancestors)
    f1: picking 'keep' action
    f2: picking 'get' action
   end of auction
@@ -230,7 +230,7 b' Verify how the output looks and and how '
   calculating bids for ancestor 40663881a6dd
   resolving manifests
   
-  auction for merging merge bids
+  auction for merging merge bids (2 ancestors)
    f1: picking 'get' action
    f2: picking 'keep' action
   end of auction
@@ -257,7 +257,7 b' Verify how the output looks and and how '
    f1: versions differ -> m
    f2: remote unchanged -> k
   
-  auction for merging merge bids
+  auction for merging merge bids (2 ancestors)
    f1: picking 'get' action
    f2: picking 'keep' action
   end of auction
@@ -343,7 +343,7 b' http://stackoverflow.com/questions/93500'
   calculating bids for ancestor b211bbc6eb3c
   resolving manifests
   
-  auction for merging merge bids
+  auction for merging merge bids (2 ancestors)
    x: multiple bids for merge action:
     versions differ -> m
     versions differ -> m
@@ -430,7 +430,7 b' Verify that the old context ancestor wor'
    d1/b: other deleted -> r
    d2/b: remote created -> g
   
-  auction for merging merge bids
+  auction for merging merge bids (2 ancestors)
    d1/a: consensus for r
    d1/b: consensus for r
    d2/b: consensus for g