New preference mergenot

[g-raud]

This preference is client-local so it does not create incompatibilities.

One use is to disable all merges on the command line with -mergenot "Regex .*", for example to avoid interactive merges in batch mode.

Another use is to selectively disable merge options in a profile after generic merge patterns at the start of the file.

[hjmjohnson]

@g-raud Please rebase on master branch to ensure that this is still relevant and clean.