tests: don't assume a .git/info for .git/info/grafts

tests: don't assume a .git/info for .git/info/grafts

Change those tests that assumed that a .git/info directory would be
created for them when writing .git/info/grafts to explicitly create
the directory.

Do this using the new "TEST_CREATE_REPO_NO_TEMPLATE" facility, and use
"mkdir" instead of "mkdir -p" to assert that we don't have the
.git/info already. An exception to this is the "with grafts" test in
"t6001-rev-list-graft.sh". There we're modifying our ".git" state in a
for-loop, in lieu of refactoring that more extensively let's use
"mkdir -p" there.

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