Home.git.mk: Avoid repetition in recursive make rules

Replace occurences of '$(MAKE) -C ...' with '$(git.make_cmd)'.
git: Fix git build

Git's Documentation/Makefile defines incomplete dependencies, which
breaks parallel make. So build documentation with -j1.
bootstrap: Fix recipe for ~/stamp/make-4.0/stow
build: Add include_sub_makefiles.mk
Home.mk: Refactor inclusion of sub makefiles

Use $(home_include_sub_makefiles_mk).