Some return codes for functions which may fail are not being checked in
`test_rebase_submodule__initialize`. This may lead us to not notice
errors when initializing the environment and would possibly result in
either memory corruption or segfaults as soon as any of the
initialization steps fails. Fix this by wrapping these function calls
into `cl_git_pass`.

An untracked file in a submodule should not prevent a rebase from
starting.  Even if the submodule's SHA is changed, and that file would
conflict with a new tracked file, it's still OK to start the rebase
and discover the conflict later.

Signed-off-by: David Turner <dturner@twosigma.com>

git_rebase_finish relies on head_detached being set, but
rebase_init_merge was only setting it when branch->ref_name was unset.
But branch->ref_name would be set to "HEAD" in the case of detached
HEAD being either implicitly (NULL) or explicitly passed to
git_rebase_init.

Test a rebase (both a merge rebase and an inmemory rebase) with a new
commit that adds files underneath a new subfolder.

Test that we can properly abort a rebase when it is initialized by a
revspec.  This ensures that we do not conflate revspecs and refnames.

When rebasing with IDs, we do not return to the `branch`,
we remain in a detached HEAD state.

Instead of `open`ing a rebase and `abort`ing that, test that we can
`abort` a rebase that has just begun with `init`.