unamend: allow unamending if allowunstable is set
I don't see why unamend should be disallowed when allowunstable is
set. By switching to rewriteutil.precheck() we fix that and get more
consistent error messages (and some additional ones).
Differential Revision: https://phab.mercurial-scm.org/D1682
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
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Running without installing::
$ make local # build for inplace usage
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