histedit: change string literals to constants in goal naming This is a part of bigger effort to refactor histedit with ultimate goal of understanding it.

run-tests: add --chg option to install and run tests using chg Because the temporary installation directory is shared between hg and chg, --chg is not allowed if --with-hg option is specified. Also, --chg option does not work on FreeBSD because "make" command is hard-coded. These limitations can be improved later. Almost all tests will fail with chg right now.

run-tests: add --with-chg option to run tests using chg Unlike --with-hg=/path/to/chg, this option allows us to start and clean up command servers in isolated environment. And we can specify the hg command as well as the chg command.