diff: remove unused import

Signed-off-by: David Aguilar <davvid@gmail.com>
cmds: consolidate LaunchDifftool and Mergetool commands

The status widget recently learned to automatically launch mergetool on
unmerged paths.  Move that feature into the LaunchDifftool command so
that we can eliminate Mergetool.

This also makes it so that the "diff" Ctrl+D shortcut also calls
mergetool on unmerged paths where previously it would call difftool.

Signed-off-by: David Aguilar <davvid@gmail.com>
status: remove redundant actions

Reuse the launch_difftool and launch_editor in the context menus instead
of recreating them.

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