tests: don't depend on template-created .git/branches

tests: don't depend on template-created .git/branches

As noted in c8a58ac5a52 (Revert "Don't create the $GIT_DIR/branches
directory on init", 2009-10-31) there was an attempt long ago in
0cc5691a8b0 (Don't create the $GIT_DIR/branches directory on init,
2009-10-30) to get rid of the legacy "branches" directory.

We should probably get rid of its creation by removing the
"templates/branches--" file. But whatever our default behavior, our
tests should be tightened up to explicitly create the .git/branches
directory if they rely on our default templates, to make the
dependency on those templates clear.

So let's amend the two tests that would fail if .git/branches wasn't
created. To do this introduce a new "TEST_CREATE_REPO_NO_TEMPLATE"
variable, which we'll set before sourcing test-lib.sh, and change the
"git clone" and "git init" commands in the tests themselves to
explicitly pass "--template=".

This way they won't get a .git/branches in either their top-level
.git, or in the ones they create. We can then amend the tests that
rely on the ".git/branches" directory existing to create it
explicitly, and to remove it after its creation.

This new "TEST_CREATE_REPO_NO_TEMPLATE" variable is a less
heavy-handed version of the "NO_SET_GIT_TEMPLATE_DIR" variable. See
a94d305bf80 (t/t0001-init.sh: add test for 'init with init.templatedir
set', 2010-02-26) for its implementation.

Unlike "TEST_CREATE_REPO_NO_TEMPLATE", this new
"TEST_CREATE_REPO_NO_TEMPLATE" variable is narrowly scoped to what the
"git init" in test-lib.sh does, as opposed to the global effect of
"NO_SET_GIT_TEMPLATE_DIR" and the setting of "GIT_TEMPLATE_DIR" in
wrap-for-bin.sh.

I experimented with adding a new "GIT_WRAP_FOR_BIN_VIA_TEST_LIB"
variable set in test-lib.sh, which would cause wrap-for-bin.sh to not
set GIT_TEMPLATE_DIR, GITPERLLIB etc, as we set those in
test-lib.sh. I think that's a viable approach, but it would interact
e.g. with the appending feature of GITPERLLIB added in
8bade1e12e2 (wrap-for-bin: make bin-wrappers chainable, 2013-07-04).

Doing so would allow us to convert the tests in t0001-init.sh that now
use "NO_SET_GIT_TEMPLATE_DIR" to simply unset "GIT_TEMPLATE_DIR" in a
sub-shell before invoking "git init" or "git clone". I think that
approach is worth pursuing, but let's table it for now. Some future
wrap-for-bin.sh refactoring can try to address it.

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