status: launch "mergetool" on unmerged paths for Ctrl+D

Teach the Ctrl+D shortcut to launch "mergetool" when a path is unmerged.
Previously, we would blindly launch "difftool", which silently fails.

Make the user experience seemless by adding automatically invoking
"mergetool" when "difftool" is requested on an unmerged path.

Signed-off-by: David Aguilar <davvid@gmail.com>