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+spaces) have not been handled as native git. Also the upcoming
rebase interactive feature in EGit needs reading/writing these files
independent from a RebaseCommand. Therefore reading and writing those
files has been refactored to the Repository class. RebaseCommand get's
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>
Test for RebaseCommand that covers comment handling in interactive mode