Merge branch 'en/make-libgit-a'

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