graft: do not try to skip rev derived from ancestor more than once (issue6024)
We check 'x in revs' in other cases, so let's do the same.
The test case credits to Tom Prince.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
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