Fix problems in t1000-branch-create

Fix problems in t1000-branch-create

There were a number of problems with t1000-branch-create:

  * It assumed that refs were not packed. I added calls to show-ref to
    spot any packed refs.

  * It reused the same name for several consecutive branch creation
    tests. This causes a domino effect if one of the subtests fail. I
    made the subtests use different names.

  * It tried to make sure that we couldn't create a branch if there
    was a broken ref by the same name. Unfortunately that test fails
    -- we don't create the branch, but we point HEAD to it. This
    causes many subsequent operations to fail. I marked this test as a
    known failure, and added some reset code so that subsequent
    subtests don't die because of this.

Signed-off-by: Karl Hasselström <kha@treskal.com>