gitweb: Fix project-specific feature override behavior

gitweb: Fix project-specific feature override behavior

This commit fixes a bug in processing project-specific override in
a situation when there is no project, e.g. for the projects list page.

When 'snapshot' feature had project specific config override enabled
by putting
  $feature{'snapshot'}{'override'} = 1;

(or equivalent) in $GITWEB_CONFIG, and when viewing toplevel gitweb
page, which means the projects list page (to be more exact this
happens for any project-less action), gitweb would put the following
Perl warnings in error log:

  gitweb.cgi: Use of uninitialized value $git_dir in concatenation (.) or string at gitweb.cgi line 2065.
  fatal: error processing config file(s)
  gitweb.cgi: Use of uninitialized value $git_dir in concatenation (.) or string at gitweb.cgi line 2221.
  gitweb.cgi: Use of uninitialized value $git_dir in concatenation (.) or string at gitweb.cgi line 2218.

The problem is in the following fragment of code:

  # path to the current git repository
  our $git_dir;
  $git_dir = "$projectroot/$project" if $project;

  # list of supported snapshot formats
  our @snapshot_fmts = gitweb_get_feature('snapshot');
  @snapshot_fmts = filter_snapshot_fmts(@snapshot_fmts);

For the toplevel gitweb page, which is the list of projects, $project is not
defined, therefore neither is $git_dir.  gitweb_get_feature() subroutine
calls git_get_project_config() if project specific override is turned
on... but we don't have project here.

Those errors mentioned above occur in the following fragment of code in
git_get_project_config():

  	# get config
  	if (!defined $config_file ||
  	    $config_file ne "$git_dir/config") {
  		%config = git_parse_project_config('gitweb');
  		$config_file = "$git_dir/config";
  	}

git_parse_project_config() calls git_cmd() which has '--git-dir='.$git_dir

There are (at least) three possible solutions:
1. Harden gitweb_get_feature() so that it doesn't call
   git_get_project_config() if $project (and therefore $git_dir) is not
   defined; there is no project for project specific config.
2. Harden git_get_project_config() like you did in your fix, returning early
   if $git_dir is not defined.
3. Harden git_cmd() so that it doesn't add "--git-dir=$git_dir" if $git_dir
   is not defined, and change git_get_project_config() so that it doesn't
   even try to access $git_dir if it is not defined.

This commit implements both 1.) and 2.), i.e. gitweb_get_feature() doesn't
call project-specific override if $git_dir is not defined (if there is no
project), and git_get_project_config() returns early if $git_dir is not
defined.

Add a test for this bug to t/t9500-gitweb-standalone-no-errors.sh test.

Reported-by: Eli Barzilay <eli@barzilay.org>
Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>