gitweb: Show appropriate "Generating..." page when regenerating cache

gitweb: Show appropriate "Generating..." page when regenerating cache

When there exist stale/expired (but not too stale) version of
(re)generated page in cache, gitweb returns stale version (and updates
cache in background, assuming 'background_cache' is set to true value).
When there is no stale version suitable to serve the client, currently
we have to wait for the data to be generated in full before showing it.
Add to GitwebCache::FileCacheWithLocking, via 'generating_info' callback,
the ability to show user some activity indicator / progress bar, to
show that we are working on generating data.

Note that without generating data in background, process generating
data wouldn't print progress info, because 'generating_info' can exit
(and in the case of gitweb's git_generating_data_html does exit).

Gitweb itself uses "Generating..." page as activity indicator, which
redirects (via <meta http-equiv="Refresh" ...>) to refreshed version
of the page after the cache is filled (via trick of not closing page
and therefore not closing connection till data is available in cache,
checked by getting shared/readers lock on lockfile for cache entry).
The git_generating_data_html() subroutine, which is used by gitweb
to implement this feature, is highly configurable: you can choose
frequency of writing some data so that connection won't get closed,
and maximum time to wait for data in "Generating..." page (see comments
in %generating_options hash definition).

The git_generating_data_html() subroutine would return early (not showing
HTML-base progress indicator) if action does not return HTML output, or
if web browser / user agent is a robot / web crawler (or gitweb is run as
standalone script).  In such cases HTML "Generating..." page does not make
much sense.

For this purpose new subrotuines action_outputs_html($action) (which uses
%actions_info introduced in earlier commit) and browser_is_robot() (which
uses HTTP::BrowserDetect if possible, and fall backs on simple check of
User-Agent string) were added.

WARNING: If 'generating_info' subroutine always exits, like
git_generating_data_html() currently does, then there would be problems
with error pages, which are not cached... unless the process generating
data does not use 'generating_info', see the next commit.  The initial
delay introduced in 2nd next commit migitates this issue somewhat.

Alternate solution would be to print output when generating data
('tee'-ing it).

Add test for simple (not exiting) 'generating_info' subroutine, for both
case with background generation disabled and enabled to t9503.

Inspired-by-code-by: John 'Warthog9' Hawley <warthog9@kernel.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>