add: add a label for messages about added files Keeping consistency between addremove and add seems a good idea.

addremove: add labels for messages about added and removed files This is the first step to add color for add and remove output.

revlog: filter out "invalid" delta base candidates There are bases that we know won't produce a good delta chain. For example, if the base is already bigger than twice the size of the text we store, we know the resulting delta chain will never be valid. We might make the check a bit more powerful and generic in the future, but this looks a good start. In particular, empty file (size 0) will never find a good base, so we should stop spending time trying to find one.

revlog: use a symbolic constant for the deltas/text ration We are about to use it somewhere else, we should as well make it easier to access and change.