Merge branch 'en/make-libgit-a'

Instead of three library archives (one for git, one for reftable, and one for xdiff), roll everything into a single libgit.a archive. This would help later effort to FFI into Rust. * en/make-libgit-a: make: delete REFTABLE_LIB, add reftable to LIB_OBJS make: delete XDIFF_LIB, add xdiff to LIB_OBJS