hgweb: use patch.diffhunks in webutil.diffs to simplify the algorithm
Function patch.diffhunks yields items for a "block" (i.e. a file) as a whole
so take advantage of this to simplify the algorithm and avoid parsing diff
lines to determine whether we're starting a new "block" or not. Thus we drop
to external block counter and rely on diffhunks iterations instead.
We also take advantage of the fact that patch.diffhunks() yields *lines* of
hunks (instead of a string) to avoid building a list that is ''.join-ed into a
string that is then split.
As lines in 'header' returned by patch.diffhunks() have no trailing new line,
we need to insert it ourselves to match template expectations.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.