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>