Better support the git staging process

By @mrtux on September 14, 2014 08:14 (imported from Google Code)

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1. We should provide "Add to index" as a separate option on the context menu (dunno if we should replace the current "Add"). Because then, e.g. for rebasing ppl. do not need to perform "Commit amend" for changed files

2. Extend the commit dialog or provide a second one which better supports the staging process and maybe also hunk selection for staging

3. Make TGitCache more configurable: Let users choose whether staged changes should be shown as modified or not; or as an alternative: maybe we need to reuse the (rarely?) "assume-valid" icon for staged but now committed changes