merge: fix unnecessary rename merges on linear update (issue631)
If one side's revision is identical to the ancestor, we skip the rest
of the copy detection logic.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.