Location of actual Mercurial executable. 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.