git-gui: Add more integration options to citool.

git-gui: Add more integration options to citool.

- Make citool return nonzero exit code if it did not commit.
- Add a mode where it does not actually commit and simply
  exits with zero code. Commit message is either disabled,
  or simply dumped to GITGUI_EDITMSG before exiting.
- Add an option to immediately start it in amend mode.

Rationale:

1) Use 'git citool --nocommit' instead of mergetool in scripts.
2) Use 'git citool --amend' to edit commits while rebasing.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>