Please compare explicitly against `None` instead of testing its boolean value:

    if MergeState.DEFAULT_BUILD_COMMAND is not None:

Also, you can be more idiomatic (and safe a line or two of code) by inverting the `if` statement:

    if MergeState.DEFAULT_BUILD_COMMAND is None:
        try:
            MergeState.DEFAULT_BUILD_COMMAND = \
                check_output(['git', 'config', 'imerge.command']).rstrip()
            return MergeState.DEFAULT_BUILD_COMMAND
        except CalledProcessError:
            return None

        return MergeState.DEFAULT_BUILD_COMMAND

Finally, please note that in the (common) case that there is no such config setting, you will call the same `git config` command every time this method is called.  You can avoid that by distinguishing somehow between "not yet initialized" and "no command configured".