dev-vcs/git: fix git-subtree.1 build failure

dev-vcs/git: fix git-subtree.1 build failure

Partial revert of commit 56312573763e794982d7a7284e7e7d1a8a9f8eaa
Some of the manpages require the full documentation stack.

Documentation build is gated behind USE=doc due to the dependencies, so
move the manpage behind the conditional as well. As a future action, ask
upstream to include git-subtree.1 with the rest of the prebuilt
Documentation.

Fixes: https://bugs.gentoo.org/687848
See-Also: https://bugs.gentoo.org/517794
Package-Manager: Portage-2.3.67, Repoman-2.3.12
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>