hgweb: add a 'linerange' parameter to webutil.diffs()

This is used to filter out hunks based on their range (with respect to 'node2'
for patch.diffhunks() call, i.e. 'ctx' for webutil.diffs()).

This is the simplest way to filter diff hunks, here done on server side. Later
on, it might be interesting to perform this filtering on client side and
expose a "toggle" action to alternate between full and filtered diff.

mercurial/hgweb/webutil.py

@@ -434,7 +434,7 b' def listfilediffs(tmpl, files, node, max'
     if len(files) > max:
         yield tmpl('fileellipses')
 
-def diffs(web, tmpl, ctx, basectx, files, style):
+def diffs(web, tmpl, ctx, basectx, files, style, linerange=None):
 
     def prettyprintlines(lines, blockno):
         for lineno, l in enumerate(lines, 1):
@@ -470,6 +470,11 b' def diffs(web, tmpl, ctx, basectx, files'
             header = header[1:]
         lines = [h + '\n' for h in header]
         for hunkrange, hunklines in hunks:
+            if linerange is not None and hunkrange is not None:
+                s1, l1, s2, l2 = hunkrange
+                lb, ub = linerange
+                if not (lb <= s2 < ub or lb < s2 + l2 <= ub):
+                    continue
             lines.extend(hunklines)
         if lines:
             yield tmpl('diffblock', parity=next(parity), blockno=blockno,