gitweb: make gitweb_check_feature a boolean wrapper

gitweb: make gitweb_check_feature a boolean wrapper

The gitweb_get_feature() function retrieves the configuration parameters
for the feature (such as the list of snapshot formats or the list of
additional actions), but it is very often used to see if feature is
enabled (which is returned as the first element in the list).

Because accepting the returned list in the scalar context by mistake
yields the number of elements in the array, which is non-zero in all
cases, such a mistake would result in a bug for the latter use, with
disabled features appearing enabled.  All existing callers that call the
function for this purpose assign the return value in the list context to
retrieve the first element, but that is only because we fixed careless
callers recently.

This adds gitweb_check_feature() as a wrapper to gitweb_get_feature() that
can be called safely in the scalar context to see if a feature is enabled
to reduce the risk of future bugs.  Callers of "get" that use the call
only to see if the feature is enabled are updated to call this wrapper.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>