Hardcode SLHC branchname.
Avoid full checkout of all the differences between two branches.

Use the merge base between the current branch and the topic branch
for the sparse checkout.
Error message in case of failed merge.
Do sparse checkout based on merge base, not diff.
Perform checkdeps at the end of checkout.

* Opt-out via --unsafe option.
Do not infinite loop.
Working support for SLHC releases.
Merge pull request #23 from ktf/master

Various improvements:

- SLHC support
- Correctly handle cms-merge-topic when branches diverged.
- Add checkdeps and assouciated opt out option.