Makefile & .gitignore: ignore & clean "git.res", not "*.res"

Makefile & .gitignore: ignore & clean "git.res", not "*.res"

Adjust the overly broad .gitignore and "make clean" rule added in
ce39c2e04ce (Provide a Windows version resource for the git
executables., 2012-05-24).

For now this is merely a correctness fix, but needed because a
subsequent commit will want to check in *.res files elsewhere in the
tree, which we shouldn't have to "git add -f".

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>