gitweb: Read repo config using 'git config -z -l'

gitweb: Read repo config using 'git config -z -l'

Change git_get_project_config to run git-config only once per
repository, without changing its signature (its calling convention).
This means for example that it returns 'true' or 'false' when called
with second argument '--bool', and not true or false value.

Instead of calling 'git config [<type>] --get gitweb.<key>' once for
each config variable, call 'git config -z -l' only once, parsing and
saving its output to %config variable.  This makes possible to add new
per repository configuration without paying cost of forking once per
variable checked.  We can now allow repository description and
repository URLs to be stored in config file without badly affecting
gitweb performance.

For now only configuration variables for 'gitweb' section are stored.

Multiple values for single configuration variable are stored as
anonymous array reference; configuration variable with no value is
stored as undef.

Converting configuration variable values to boolean or integer value
are done in Perl.  Results differ from git-config in the fact that no
conversion error is ever raised.  For boolean values no value, 'true'
(any case) and 'false' (any case) are considered true, numbers are
true if not zero; all other values (even invalid for bool) are
considered false.  For integer values value suffix of 'k', 'm', or 'g'
following decimal number will cause the value to be multiplied by
1024, 1048576, or 1073741824; other values are returned as-is, only
whitespace stripped.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>