tests: rename test-merge3 to test-add-remove.
It tests that 'hg commit' does not crash if the user removes a newly
added file, which has nothing to do with merging.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.