Adapt .gitignore (re #8545)

This adapts the top-level .gitignore file to match the new situation of
base.git, ghc-prim.git, integer-gmp.git, integer-simple.git, and
template-haskell.git being folded into ghc.git

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>