merge: tell the user when we are using bid merge

Bid merge is a new rarely used feature that the user explicitly enabled - we
should tell/warn when the user actually is using it, just like we tell when we
not are using it.

Give a message like
  note: merging 3b08d01b0ab5+ and adfe50279922 using bids from ancestors 0f6b37dbe527 and 40663881a6dd

mercurial/merge.py

@@ -730,6 +730,10 b' def calculateupdates(repo, wctx, mctx, a'
                                 partial, acceptremote, followcopies)
 
     else: # only when merge.preferancestor=* - experimentalish code
+        repo.ui.status(
+            _("note: merging %s and %s using bids from ancestors %s\n") %
+            (wctx, mctx, _(' and ').join(str(anc) for anc in ancestors)))
+
         # Call for bids
         fbids = {} # mapping filename to list af action bids
         for ancestor in ancestors:

tests/test-merge-criss-cross.t

@@ -128,6 +128,7 b' Redo merge with merge.preferancestor="*"'
   $ rm f*
   $ hg up -qC .
   $ hg merge -v --debug --tool internal:dump 5 --config merge.preferancestor="*"
+  note: merging 3b08d01b0ab5+ and adfe50279922 using bids from ancestors 0f6b37dbe527 and 40663881a6dd
   
   calculating bids for ancestor 0f6b37dbe527
     searching for copies back to rev 3
@@ -172,6 +173,7 b' The other way around:'
         alternatively, use --config merge.preferancestor=40663881a6dd
   2 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ hg merge -v --debug --config merge.preferancestor="*"
+  note: merging adfe50279922+ and 3b08d01b0ab5 using bids from ancestors 0f6b37dbe527 and 40663881a6dd
   
   calculating bids for ancestor 0f6b37dbe527
     searching for copies back to rev 3
@@ -212,11 +214,13 b' Verify how the output looks and and how '
 
   $ hg up -qC
   $ hg merge --config merge.preferancestor="*"
+  note: merging 3b08d01b0ab5+ and adfe50279922 using bids from ancestors 0f6b37dbe527 and 40663881a6dd
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   (branch merge, don't forget to commit)
 
   $ hg up -qC
   $ hg merge -v --config merge.preferancestor="*"
+  note: merging 3b08d01b0ab5+ and adfe50279922 using bids from ancestors 0f6b37dbe527 and 40663881a6dd
   
   calculating bids for ancestor 0f6b37dbe527
   resolving manifests
@@ -235,6 +239,7 b' Verify how the output looks and and how '
 
   $ hg up -qC
   $ hg merge -v --debug --config merge.preferancestor="*"
+  note: merging 3b08d01b0ab5+ and adfe50279922 using bids from ancestors 0f6b37dbe527 and 40663881a6dd
   
   calculating bids for ancestor 0f6b37dbe527
     searching for copies back to rev 3
@@ -316,6 +321,7 b' http://stackoverflow.com/questions/93500'
   $ hg up -qC .
 
   $ hg merge -v --config merge.preferancestor="*"
+  note: merging 0d355fdef312+ and 4b8b546a3eef using bids from ancestors 70008a2163f6 and b211bbc6eb3c
   
   calculating bids for ancestor 70008a2163f6
   resolving manifests