merge: show the scary multiple ancestor hint for merges only, not for updates

Updates with uncommited changes will always only have one ancestor - the parent
revision. Updates between existing revision should (and will) always give the
same result no matter which ancestor is used. The warning is thus only relevant
when doing a "real" merge.

mercurial/merge.py

@@ -1052,7 +1052,7 b' def update(repo, node, branchmerge, forc'
                 cahs = repo.changelog.commonancestorsheads(p1.node(), p2.node())
                 pas = [repo[anc] for anc in (sorted(cahs) or [nullid])]
             else:
-                pas = [p1.ancestor(p2, warn=True)]
+                pas = [p1.ancestor(p2, warn=branchmerge)]
 
         fp1, fp2, xp1, xp2 = p1.node(), p2.node(), str(p1), str(p2)
 

tests/test-merge-criss-cross.t

@@ -24,8 +24,6 b' Criss cross merging'
   $ hg ci -m '5 second change f1'
 
   $ hg up -r3
-  note: using 0f6b37dbe527 as ancestor of adfe50279922 and cf89f02107e5
-        alternatively, use --config merge.preferancestor=40663881a6dd
   1 files updated, 0 files merged, 0 files removed, 0 files unresolved
   $ echo '6 second change' > f2
   $ hg ci -m '6 second change f2'
@@ -169,8 +167,6 b' Redo merge with merge.preferancestor="*"'
 The other way around:
 
   $ hg up -C -r5
-  note: using 0f6b37dbe527 as ancestor of 3b08d01b0ab5 and adfe50279922
-        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