remove 'init-branch' command

Use 'branch --init' instead...
Remove warning messages
Update release note
Update doc
Merge pull request #393 from pmiossec/remove_init_branch

Remove deprecated 'init-branch' command
Use libGit2Sharp to merge branches
throw an exception if commit not found
Update release note
Merge pull request #601 from pmiossec/libgit2sharp_merge

Use Libgit2sharp for merges