hgweb: support disabling page cache

By default, hgweb_mod supports caching via the ETag header. This can
cause some confusion with browsers which cache aggressively. This change
preserves existing behavior while giving the administrator a knob to
disable the ETag header.

doc/hgrc.5.txt

@@ -1052,6 +1052,8 The full set of options is:
 
     To disable SSL verification temporarily, specify ``--insecure`` from
     command line.
+``cache``
+    Whether to support caching in hgweb. Defaults to True.
 ``contact``
     Name or email address of the person in charge of the repository.
     Defaults to ui.username or ``$EMAIL`` or "unknown" if unset or empty.

mercurial/hgweb/hgweb_mod.py

@@ -188,7 +188,8 class hgweb(object):
                 req.form['cmd'] = [tmpl.cache['default']]
                 cmd = req.form['cmd'][0]
 
-            caching(self, req) # sets ETag header or raises NOT_MODIFIED
+            if self.configbool('web', 'cache', True):
+                caching(self, req) # sets ETag header or raises NOT_MODIFIED
             if cmd not in webcommands.__all__:
                 msg = 'no such method: %s' % cmd
                 raise ErrorResponse(HTTP_BAD_REQUEST, msg)