Different max values per type/class (esp. with vcsdiff)

[blueyed]

I have just wondered why no vcsdiff signs were displayed in one buffer, and found out about g:quickfixsigns_max.

I think it would be reasonable to have a higher value for the vcsdiff class by default and/or have an option to configure this per class.

Also, with debugging enabled, a message should get displayed if signs are not being placed at all.

It might also be better to display the maximum number of signs instead of none at all when the maximum value is reached.