rebase: fix rebase with no common ancestors (issue4446)
The new rebase revset didn't check for the case when there are no common
ancestors. Now it does. The new behavior should be the same as the pre-3.2
behavior. Added a test.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
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