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.

destutil: add more precise error classes for destmerge Having exception classes more precise than 'Abort' will allow us to properly catch "nothing to rebase" situations when we will be using 'destmerge' in rebase.