pull: implement skeletal builtin pull

pull: implement skeletal builtin pull

For the purpose of rewriting git-pull.sh into a C builtin, implement a
skeletal builtin/pull.c that redirects to $GIT_EXEC_PATH/git-pull.sh if
the environment variable _GIT_USE_BUILTIN_PULL is not defined. This
allows us to fall back on the functional git-pull.sh when running the
test suite for tests that depend on a working git-pull implementation.

This redirection should be removed when all the features of git-pull.sh
have been re-implemented in builtin/pull.c.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>