VC top of the tree diff and log

[Dan Nicolaescu]

It is quite useful to see all the changes that are present in the tree,
not just the ones for the current file (that one gets by doing C-x v =
when editing a file). 

Currently the way to get a diff from the top of the tree is to:
C-x v d TOP_OF_THE_TREE_DIRECTORY RET
put the cursor on the ./ line
C-x v = 

It would be nice to have an easier way to do this.

Similar for showing the log.

Key bindings:
for log we could have C-x v L  (should be intuitive given that
vc-print-log uses C-x v l)

not sure what would be a good one for diff

Older VCS don't have the top of the tree concept, not sure what to do
about those, but we can just ignore them for the time being.

Opinions?