branch: check for bisects and rebases

branch: check for bisects and rebases

The branch_checked_out() helper was added by the previous change, but it
used an over-simplified view to check if a branch is checked out. It
only focused on the HEAD symref, but ignored whether a bisect or rebase
was happening.

Teach branch_checked_out() to check for these things, and also add tests
to ensure that we do not lose this functionality in the future.

Now that this test coverage exists, we can safely refactor
validate_new_branchname() to use branch_checked_out().

Note that we need to prepend "refs/heads/" to the 'state.branch' after
calling wt_status_check_*(). We also need to duplicate wt->path so the
value is not freed at the end of the call.

Signed-off-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>