subrepo: disallow symlink traversal across subrepo mount point (SEC)
It wasn't easy to extend the pathauditor to check symlink traversal across
subrepos because pathauditor._checkfs() rejects a directory having ".hg"
directory. That's why I added the explicit islink() check.
No idea if this patch is necessary after we've fixed the issue5730 by
splitting submerge() into planning and execution phases.
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