manifestmerge: local unknown, remote created: don't traverse symlinks

To figure out what to do with locally unknown files, Mercurial attempts to read
them if they exist. When an attempt is made to read a file that exists but
traverses a symlink, Mercurial aborts.

With this patch, we first ensure that the file doesn't traverse a symlink
before opening it. This is fine because a file being "remote created" means the
symlink doesn't exist remotely, which means it will be deleted in the apply
phase.

mercurial/merge.py

@@ -95,6 +95,7 b' class mergestate(object):'
 def _checkunknownfile(repo, wctx, mctx, f):
     return (not repo.dirstate._ignore(f)
         and os.path.isfile(repo.wjoin(f))
+        and repo.wopener.audit.check(f)
         and repo.dirstate.normalize(f) not in repo.dirstate
         and mctx[f].cmp(wctx[f]))
 

tests/test-symlinks.t

@@ -160,6 +160,15 b' now addremove should remove old files'
   adding bar/a
   adding foo
   removing foo/a
+
+commit and update back
+
+  $ hg ci -mb
+  $ hg up '.^'
+  1 files updated, 0 files merged, 2 files removed, 0 files unresolved
+  $ hg up tip
+  2 files updated, 0 files merged, 1 files removed, 0 files unresolved
+
   $ cd ..
 
 == root of repository is symlinked ==