sparse-checkout: create leading directory

sparse-checkout: create leading directory

When creating the sparse-checkout file, Git does not create the leading
directory, "$GIT_DIR/info", if it does not exist. This causes problems
if the repository does not have that directory. Therefore, ensure that
the leading directory is created.

This is the only "open" in builtin/sparse-checkout.c that does not have
a leading directory check. (The other one in write_patterns_and_update()
does.)

Note that the test needs to explicitly specify a template when running
"git init" because the default template used in the tests has the
"info/" directory included.

Helped-by: Jose Lopes <jabolopes@google.com>
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>