drawdag: don't crash when writing copy info to changesets
When writing copies to the changeset, localrepo.commitctx() will call
ctx.p1copies() and ctx.p2copies(). These crashed on simplecommitctx
because they ended up trying to access the manifest. drawdag doesn't
support copies at all, so we can simply override the methods to return
empty dicts.
Differential Revision: https://phab.mercurial-scm.org/D6565
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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$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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