Major improvements.

git cms-merge-topic:

- Less embarassing option handling
- Does not get confused by .gitignore, now present in CMSSW.
- Support for non pull request branches.
- Support for non cms-sw repositories.

git cms-addpkg:

- Brown paperbag bug which result in git cms-addpkg reverting changes pulled
  in.

git cms-checkdeps:

- Uses $CMSSW_VERSION as comparison base, regardless of the tag used by git
  cms-addpkg.
Updated man page for git-cms-merge-topic.
Standalone setup script.
Introduce a new helper git-cms-sparse-checkout.

This is done to reuse code between git-cms-merge-topic and git-cms-addpkg.

Notice it does not do any extra check, nor it invokes any git read-tree, but it
leaves that task to the caller.