gitweb: Silence 'Variable VAR may be unavailable' warnings

gitweb: Silence 'Variable VAR may be unavailable' warnings

When $projects_list points to a directory, and git_get_projects_list
scans this directory for repositories, there can be generated the
following warnings (for persistent services like mod_perl or plackup):

  Variable "$project_maxdepth" may be unavailable at gitweb.cgi line 2443.
  Variable "$projectroot" may be unavailable at gitweb.cgi line 2451.

Those are false positives; silence those warnings by explicitely
declaring $project_maxdepth and $projectroot with 'our', as global
variables, in anonymous subrotine passed to File::Find::find.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>