hgweb: use revset.spanset where appropriate

It is remainder of 9ad6dae67845 where spanset was introduced.

mercurial/hgweb/webcommands.py

@@ -187,7 +187,7 b' def _search(web, req, tmpl):'
 
         mfunc = revset.match(web.repo.ui, revdef)
         try:
-            revs = mfunc(web.repo, revset.baseset(web.repo))
+            revs = mfunc(web.repo, revset.spanset(web.repo))
             return MODE_REVSET, revs
             # ParseError: wrongly placed tokens, wrongs arguments, etc
             # RepoLookupError: no such revision, e.g. in 'revision:'