[RFC] imerge wrapper for merge and resolve

[Simon Heimberg]

Some wrapper code for imerge around merge and resolve. It needs the
patches I have sent because --tool is used.

All commands print the command which should be used instead.
Unsupported options and commands are marked with (DOES NOT WORK) in the
documentation. When an unsupported option is used, a warning is printed.
Unsupported commands do an alert.
If the wrapper should do more, this should be implemented in merge or
resolve first. Maybe the option to merge the next unresolved file is
useful. When are load and save used? Is --auto the same as "--config
ui.interactive=False"?

The code is attached because it looks ugly as a patch.

Greetings,
S. Heimberg