update: just merge unknown file collisions

The unknown file collision rule was introduced as an extension of the
"should be clean when merging" rule. Unfortunately, it got applied to
the normal update path, which should be happy to merge local changes.

This patch gives us merges for unknown file collisions on update,
while preserving abort for merge and update -c.

mercurial/commands.py

@@ -5723,6 +5723,7 b' def update(ui, repo, node=None, rev=None'
             raise util.Abort(_("uncommitted local changes"))
         if not rev:
             rev = repo[repo[None].branch()].rev()
+        mergemod._checkunknown(repo, repo[None], repo[rev])
 
     if clean:
         ret = hg.clean(repo, rev)

mercurial/merge.py

@@ -545,6 +545,8 b' def update(repo, node, branchmerge, forc'
             if not force and (wc.files() or wc.deleted()):
                 raise util.Abort(_("outstanding uncommitted changes"),
                                  hint=_("use 'hg status' to list changes"))
+            if not force:
+                _checkunknown(repo, wc, p2)
             for s in wc.substate:
                 if wc.sub(s).dirty():
                     raise util.Abort(_("outstanding uncommitted changes in "
@@ -566,8 +568,6 b' def update(repo, node, branchmerge, forc'
         ### calculate phase
         action = []
         folding = not util.checkcase(repo.path)
-        if not force:
-            _checkunknown(repo, wc, p2)
         if folding:
             _checkcollision(p2, branchmerge and p1)
         action += _forgetremoved(wc, p2, branchmerge)