web: provide diff summary to the changeset page
This is the same message displayed at the end of the "diff --stat" command.
For example, "9 files changed, 1651 insertions(+), 2 deletions(-)".
The webutil.diffstatgen function allows the diffstat data to be lazily
calculated only once and then re-used.
workingfilectx.date = (1000, 0)
ASCII : Gr?ezi!
Latin-1 : Grüezi!
UTF-8 : Grüezi!