hgweb: use latest mtime for caching tag (issue4814)

Previously, we were using the last mtime we saw when reporting the
HTTP cache etag. When we added bookmarks to the end of the list of
files checked, unchanged or missing bookmarks would keep the client
cache from being invalidated.

mercurial/hgweb/hgweb_mod.py

@@ -131,11 +131,13 b' class hgweb(object):'
 
     def refresh(self, request=None):
         repostate = []
+        mtime = 0
         # file of interrests mtime and size
         for meth, fname in foi:
             prefix = getattr(self.repo, meth)
             st = get_stat(prefix, fname)
             repostate.append((st.st_mtime, st.st_size))
+            mtime = max(mtime, st.st_mtime)
         repostate = tuple(repostate)
         # we need to compare file size in addition to mtime to catch
         # changes made less than a second ago
@@ -153,7 +155,7 b' class hgweb(object):'
             # update these last to avoid threads seeing empty settings
             self.repostate = repostate
             # mtime is needed for ETag
-            self.mtime = st.st_mtime
+            self.mtime = mtime
         if request:
             self.repo.ui.environ = request.env