mergetools: vimdiff issue a warning explaining how to abort
Adds a message displayed at each vimdiff invocation:
merge conflict detected, type ":cq" to abort
Vimdiff is very confusing for non-vim user (not to speak about vim user confused anyway. However it is very likely that vimdiff is picked as the mergetool of choice when using the default config: - vim is available on all UNIX system. - Its one of the rare non graphical merge tools.