git, subtree, xmlto and docbook

[Grzegorz Junka]

Enabling git-subtree in git 2.11.0_4 adds dependency on textproc/xmlto, 
which in turns add dependency on the whole textproc/docbook chain even 
if no options in xmlto are enabled. Why do I need docbook to use 
git-subtree?

Grzegorz