Enhance reading of git-rebase-todo formatted files

Enhance reading of git-rebase-todo formatted files

Reading and writing files formatted like the git-rebase-todo files was
hidden in the RebaseCommand. Certain constructs (like leading tabs and
spaces) have not been handled as in native git. Also the upcoming
rebase interactive feature in EGit needs reading/writing these files
independently from a RebaseCommand.

Therefore reading and writing those files has been moved to the
Repository class. RebaseCommand gets smaller because of that and doesn't
have to deal with reading/writing files.

Additional tests for empty todo-list files, or files containing comments
have been added.

Change-Id: I323f3619952fecdf28ddf50139a88e0bea34f5ba
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Also-by: Tobias Pfeifer <to.pfeifer@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>