gitweb: Fix warnings with override permitted but no repo override

gitweb: Fix warnings with override permitted but no repo override

When a feature like "blame" is permitted to be overridden in the
repository configuration but it is not actually set in the repository,
a warning is emitted due to the undefined value of the repository
configuration, even though it's a perfectly normal condition.
Emitting warning is grounds for test failure in the gitweb test
script.

This error was caused by rewrite of git_get_project_config from using
"git config [<type>] <name>" for each individual configuration
variable checked to parsing "git config --list --null" output in
commit b201927 (gitweb: Read repo config using 'git config -z -l').
Earlier version of git_get_project_config was returning empty string
if variable do not exist in config; newer version is meant to return
undef in this case, therefore change in feature_bool was needed.

Additionally config_to_* subroutines were meant to be invoked only if
configuration variable exists; therefore we added early return to
git_get_project_config: it now returns no value if variable does not
exists in config.  Otherwise config_to_* subroutines (config_to_bool
in paryicular) wouldn't be able to distinguish between the case where
variable does not exist and the case where variable doesn't have value
(the "[section] noval" case, which evaluates to true for boolean).

While at it fix bug in config_to_bool, where checking if $val is
defined (if config variable has value) was done _after_ stripping
leading and trailing whitespace, which lead to 'Use of uninitialized
value' warning.

Add test case for features overridable but not overriden in repo
config, and case for no value boolean configuration variable.

Signed-off-by: Marcel M. Cary <marcel@oak.homeunix.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>