core: Add preconditions to generator function

This helps static analysis.
git.mk: ignore .deps and all .o files

Fixes

 Untracked files:
       src/.deps/
       src/src_hitori-generator.o
       src/src_hitori-interface.o
       src/src_hitori-main.o
       src/src_hitori-rules.o

Proposed for upstream at https://github.com/behdad/git.mk/issues/15

https://bugzilla.gnome.org/show_bug.cgi?id=731309