Implement "move_repo_to_merging_state". Resolving conflicts not implemented

[olsen232]

Running "sno merge <other_branch> will merge other branch, unless there are conflicts.
In which case, it will put the repository into a merging state.

~/mysnorepo (BARE:mybranch) $ sno merge other
~/mysnorepo (BARE:mybranch|MERGING) $

Unfortunately, since conflicts cannot be resolved, the only option is to abandon the merging state.

~/mysnorepo (BARE:mybranch|MERGING) $ sno merge --abort
~/mysnorepo (BARE:mybranch) $

However, the merging state is real, and contains (most of?) the information we will need to make step-by-step resolution of conflicts possible. Resolving conflicts is to be done in a follow up PR.

Also still to be done is a way to display the merging state including conflicts / summary of conflicts. Running sno status and sno show right now doesn't mention that you are in merging state. This is to be done in a follow up PR.
Other commands may also have to be deactivated, since they make no sense in the context of a merge.

Some tidying:

• Added a new file sno/repo_files.py to make it easier to read and write files inside the bare repository. Modified commit.py to use this functionality.
• Changed the JSON output of merge.py to focus on commits, not commits + branches. Commits are what is actually merged, and the branch head may have moved on by the the merge is complete.

[rcoup]

LGTM