CommitDlg: Do not use libgit2 index manipulation for commit by default any more

Issues described in commit.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
MassiveGitTask: Allow to run w/o error handling

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Use MassiveGitTask for index manipulation

Signed-off-by: Sven Strickroth <email@cs-ware.de>
RebaseDlg: Improved error handling on conflict handling

Signed-off-by: Sven Strickroth <email@cs-ware.de>