INSTALLATION will be substituted with installation directory.
Normally INSTALLATION/bin/hg but you could specify another name
such as INSTALLATION/my-mercurial.py or even look for Mercurial
in the default path (leaving installation directory blank) using just hg.
If you are configuring per-node tool home locations,
you may leave the installation directory undefined on some nodes,
in which case just hg (i.e., from $PATH) is used.