Git: Support staging a single chunk.

Add context menu in diff editor to stage/unstage a single chunk from the diff.

Task-number: QTCREATORBUG-5875
Change-Id: Ic244a0d84b5ed5f66b90d7fe8784fc1b8041d183
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Tobias Hunger <tobias.hunger@digia.com>